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/Utilities/SystemUtilities.cs

@@ -0,0 +1,42 @@
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Windows.Storage;
+using Windows.Storage.Pickers;
+using WinRT.Interop;
+
+namespace Ghost.App.Utilities;
+
+public static class SystemUtilities
+{
+    public static async Task<StorageFolder?> OpenFolderPickerAsync(PickerLocationId startLocation = PickerLocationId.DocumentsLibrary, string settingsIdentifier = "")
+    {
+        var openPicker = new FolderPicker();
+        var hWnd = WindowNative.GetWindowHandle(GhostApplication.Window);
+        InitializeWithWindow.Initialize(openPicker, hWnd);
+
+        openPicker.SuggestedStartLocation = startLocation;
+        openPicker.FileTypeFilter.Add("*");
+        openPicker.SettingsIdentifier = settingsIdentifier;
+
+        var folder = await openPicker.PickSingleFolderAsync();
+        return folder;
+    }
+
+    public static async Task<StorageFile?> OpenFilePickerAsync(PickerLocationId startLocation = PickerLocationId.DocumentsLibrary, string settingsIdentifier = "", params IEnumerable<string> filter)
+    {
+        var openPicker = new FileOpenPicker();
+        var hWnd = WindowNative.GetWindowHandle(GhostApplication.Window);
+        InitializeWithWindow.Initialize(openPicker, hWnd);
+
+        openPicker.SuggestedStartLocation = startLocation;
+        openPicker.SettingsIdentifier = settingsIdentifier;
+        foreach (var fileType in filter)
+        {
+            openPicker.FileTypeFilter.Add(fileType);
+        }
+
+        var file = await openPicker.PickSingleFileAsync();
+        return file;
+    }
+}