Make sure Application.ResourcesChanged is raised.

Previously if `Application.ResourcesChanged` was null when `Resources` was created, adding the handler via `_resources.ResourcesChanged += ResourcesChanged` did nothing because `ResourcesChanged` was null.

src/Avalonia.Controls/Application.cs

@@ -122,11 +122,11 @@ namespace Avalonia
                 if (_resources != null)
                 {
                     hadResources = _resources.Count > 0;
-                    _resources.ResourcesChanged -= ResourcesChanged;
+                    _resources.ResourcesChanged -= ThisResourcesChanged;
                 }
 
                 _resources = value;
-                _resources.ResourcesChanged += ResourcesChanged;
+                _resources.ResourcesChanged += ThisResourcesChanged;
 
                 if (hadResources || _resources.Count > 0)
                 {
@@ -343,5 +343,10 @@ namespace Avalonia
                 .Bind<IGlobalClock>().ToConstant(clock)
                 .GetService<IRenderLoop>()?.Add(clock);
         }
+
+        private void ThisResourcesChanged(object sender, ResourcesChangedEventArgs e)
+        {
+            ResourcesChanged?.Invoke(this, e);
+        }
     }
 }