Add dirty UI element visualization to UISystem, fix Frame being constantly updated.

Voile/Source/Resources/Resource.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics.CodeAnalysis;
 using Voile.Resources;
 
 namespace Voile
@@ -32,7 +33,7 @@ namespace Voile
         /// </summary>
         /// <param name="value">An instance of a retrieved <see cref="Resource"/>.</param>
         /// <returns><c>true</c> if the resource was successfully retrieved, otherwise <c>false</c>.</returns>
-        public bool TryGetValue(out T? value)
+        public bool TryGetValue([NotNullWhen(true)] out T? value)
         {
             value = ResourceManager.GetResource<T>(Guid);
             return value != null;