Refactor project from Ghost.App to Ghost.Editor

Changed the project structure to reflect a shift from `Ghost.App` to `Ghost.Editor`, updating namespaces and class names throughout.

Changed the application class in `App.xaml` and `App.xaml.cs` from `GhostApplication` to `EditorApplication`.

Changed several service interfaces to reside under `Ghost.Editor.Services.Contracts`, including `IInspectorService`, `INotificationService`, and `IProgressService`.

Added `InspectorView` and `InspectorViewModel` classes to manage inspector functionality.

Added `NavigationTabView` and `NavigationTabPage` classes to facilitate navigation within the editor.

Enhanced `WorldNode` and `EntityNode` classes to support scene graph functionality, including serialization and entity management.

Updated the project file `Ghost.Editor.csproj` to reflect the new structure and removed old references.

Modified the solution file `GhostEngine.sln` to remove references to `Ghost.App` and include `Ghost.Editor`.

Updated unit tests to align with the new namespaces and project structure.

Ghost.App/Core/AppState/IAppState.cs

@@ -0,0 +1,28 @@
+using System.Threading.Tasks;
+
+namespace Ghost.Editor.Core.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);
+}