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.Editor/Models/Asset.cs

@@ -0,0 +1,24 @@
+namespace Ghost.Editor.Models;
+
+public abstract class Asset
+{
+    /// <summary>
+    /// Get the Guid of the asset.
+    /// </summary>
+    public Guid GUID
+    {
+        get;
+    } = Guid.NewGuid();
+
+    /// <summary>
+    /// True if the asset is a folder, false if it is a file.
+    /// </summary>
+    public bool IsFolder
+    {
+        get;
+    }
+
+    internal void GenerateMetadata()
+    {
+    }
+}

Ghost.Editor/Models/MessageType.cs

@@ -0,0 +1,9 @@
+namespace Ghost.Editor.Models;
+
+public enum MessageType
+{
+    Informational,
+    Success,
+    Warning,
+    Error
+}

Ghost.Editor/Models/WorldAsset.cs

@@ -0,0 +1,14 @@
+using Ghost.Editor.AssetHandle;
+using Ghost.Editor.Resources;
+using Ghost.Editor.SceneGraph;
+
+namespace Ghost.Editor.Models;
+
+public class WorldAsset : Asset
+{
+    [AsyncAssetOpenHandler(FileExtensions.SCENE_FILE_EXTENSION)]
+    public static async Task Open(string path)
+    {
+        await EditorWorldManager.LoadWorld(path);
+    }
+}