Add game scene and level catalog interfaces, and implement scene management in AppRoot

app/IGameScenes.cs

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+using Godot;
+
+namespace Mr.BrickAdventures.app;
+
+public interface IGameScenes
+{
+    void Load(PackedScene scene);
+    void Restart();
+    void ReturnToMain(PackedScene mainMenu);
+}

app/IGameScenes.cs.uid

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+uid://b7d1ospa5p4nx

app/ILevelCatalog.cs

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+using Godot;
+
+namespace Mr.BrickAdventures.app;
+
+public interface ILevelCatalog
+{
+    int Count { get; }
+    PackedScene? Get(int index);
+    PackedScene First { get; }
+    PackedScene MainMenu { get; }
+}

app/ILevelCatalog.cs.uid

@@ -0,0 +1 @@
+uid://b2sxttgvk6nuq