Refactor application structure and add unit tests

Added:
- New `ProgressService` class for managing progress indicators.
- New `AssetDatabase`, `AssetOpenHandlerAttribute`, and `AsyncAssetOpenHandlerAttribute` classes for asset handling.
- `Ghost.UnitTest` project for unit testing with associated files and configurations.

Changed:
- `ActivationHandler` class to ensure correct handling of `LaunchActivatedEventArgs`.
- `App.xaml.cs` to register `INotificationService` and `IProgressService`, replacing `StackedNotificationService`.
- `OnLaunched` method in `App.xaml.cs` to correctly call `ActivationHandler.Handle(args)` and start the host.
- `INavigationAware` interface from internal to public for broader access.
- `EditorState.cs` to activate `EditorApplication` with the current service provider.
- Property names in `AssetItem` and `ExplorerItem` structs to `Name` and `FullName`.
- `NotificationService` class to implement `INotificationService` and refactor notification handling.
- `AssetPathToGlyphConverter` to handle file extensions consistently.
- Bindings in `ProjectPage.xaml` and `ProjectPage.xaml.cs` to use `FullName` instead of `Path`.
- `EngineEditorWindow` and `LandingWindow` classes to utilize new notification and progress services.
- `Logger` class to include a new method for logging errors with exceptions.

Updated:
- Manifest files and project files to reflect new structure and dependencies.
- Solution file `GhostEngine.sln` to include the new unit test project.
- Added several new test classes and methods in `UnitTests.cs`.

Ghost.App/Infrastructures/AppState/AppStateMachine.cs

@@ -0,0 +1,38 @@
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+
+namespace Ghost.App.Infrastructures.AppState;
+
+internal class AppStateMachine
+{
+    private Dictionary<StateKey, Lazy<IAppState>> s_states = new();
+    private IAppState? s_current;
+
+    public void RegisterState(StateKey key, Func<IAppState> stateFactory)
+    {
+        s_states[key] = new(stateFactory);
+    }
+
+    public async Task TransitionToAsync(StateKey stateKey, object? parameter = null)
+    {
+        var previous = s_current;
+        var next = s_states[stateKey].Value;
+
+        if (previous != null)
+        {
+            await previous.OnExitingAsync();
+        }
+
+        await next.OnEnteringAsync(parameter);
+
+        if (previous != null)
+        {
+            await previous.OnExitedAsync();
+        }
+
+        await next.OnEnteredAsync(parameter);
+
+        s_current = next;
+    }
+}

Ghost.App/Infrastructures/AppState/EditorState.cs

@@ -0,0 +1,65 @@
+using Ghost.App.View.Windows;
+using Ghost.Data.Models;
+using Ghost.Data.Services;
+using Ghost.Editor;
+using Ghost.Engine;
+using Microsoft.UI.Xaml;
+using System;
+using System.Threading.Tasks;
+
+namespace Ghost.App.Infrastructures.AppState;
+
+internal class EditorState : IAppState
+{
+    private EngineEditorWindow? _window;
+    private EngineCore? _engineCore;
+
+    public Task OnExitingAsync()
+    {
+        if (GhostApplication.Window == _window)
+        {
+            GhostApplication.Window = null;
+        }
+        return Task.CompletedTask;
+    }
+
+    public async Task OnEnteringAsync(object? parameter)
+    {
+        if (parameter is not ProjectMetadataInfo metadataInfo)
+        {
+            throw new ArgumentException("Parameter must be of type ProjectMetadata.", nameof(parameter));
+        }
+
+        ProjectService.CurrentProject = metadataInfo;
+
+        _engineCore = GhostApplication.GetService<EngineCore>();
+        await _engineCore.StartAsync(new Engine.Models.LaunchArgument());
+
+        _window = GhostApplication.GetService<EngineEditorWindow>();
+        _window.Activate();
+
+        GhostApplication.Window = _window;
+    }
+
+    public async Task OnExitedAsync()
+    {
+        if (_engineCore != null)
+        {
+            await _engineCore.ShutDownAsync();
+        }
+
+        if (GhostApplication.Window == _window)
+        {
+            GhostApplication.Window = null;
+        }
+
+        _window?.Close();
+        _window = null;
+    }
+
+    public Task OnEnteredAsync(object? parameter)
+    {
+        EditorApplication.Activate(parameter, ((GhostApplication)(Application.Current)).Host.Services);
+        return Task.CompletedTask;
+    }
+}

Ghost.App/Infrastructures/AppState/IAppState.cs

@@ -0,0 +1,28 @@
+using System.Threading.Tasks;
+
+namespace Ghost.App.Infrastructures.AppState;
+
+internal interface IAppState
+{
+    /// <summary>
+    /// Called when exiting the state.
+    /// </summary>
+    public Task OnExitingAsync();
+
+    /// <summary>
+    /// Called when entering the state, right after OnEnteringAsync.
+    /// <paramref name="parameter">can be used to pass data into the state, such as a project to load.</summary>
+    /// </summary>
+    public Task OnEnteringAsync(object? parameter);
+
+    /// <summary>
+    /// Called when exiting the state, specifically for pose transitions.
+    /// </summary>
+    public Task OnExitedAsync();
+
+    /// <summary>
+    /// Called when entered the state, specifically after the state has been fully initialized and is ready for interaction.
+    /// </summary>
+    /// <param name="parameter">can be used to pass data into the state, such as a project to load.</param>
+    public Task OnEnteredAsync(object? parameter);
+}

Ghost.App/Infrastructures/AppState/LandingState.cs

@@ -0,0 +1,44 @@
+using Ghost.App.View.Windows;
+using System.Threading.Tasks;
+
+namespace Ghost.App.Infrastructures.AppState;
+
+internal class LandingState : IAppState
+{
+    private LandingWindow? _window;
+
+    public Task OnExitingAsync()
+    {
+        if (GhostApplication.Window == _window)
+        {
+            GhostApplication.Window = null;
+        }
+        return Task.CompletedTask;
+    }
+
+    public Task OnEnteringAsync(object? parameter)
+    {
+        _window = GhostApplication.GetService<LandingWindow>();
+        GhostApplication.Window = _window;
+
+        _window.Activate();
+        return Task.CompletedTask;
+    }
+
+    public Task OnExitedAsync()
+    {
+        if (GhostApplication.Window == _window)
+        {
+            GhostApplication.Window = null;
+        }
+
+        _window?.Close();
+        _window = null;
+        return Task.CompletedTask;
+    }
+
+    public Task OnEnteredAsync(object? parameter)
+    {
+        return Task.CompletedTask;
+    }
+}

Ghost.App/Infrastructures/AppState/StateKey.cs

@@ -0,0 +1,8 @@
+namespace Ghost.App.Infrastructures.AppState;
+
+internal enum StateKey
+{
+    None,
+    Landing,
+    EngineEditor,
+}