Make resource dictionary keys objects.

For parity with other XAML frameworks.

src/Avalonia.Styling/Controls/IResourceDictionary.cs

@@ -9,7 +9,7 @@ namespace Avalonia.Controls
     /// <summary>
     /// An indexed dictionary of resources.
     /// </summary>
-    public interface IResourceDictionary : IResourceProvider, IDictionary<string, object>
+    public interface IResourceDictionary : IResourceProvider, IDictionary<object, object>
     {
         /// <summary>
         /// Gets a collection of child resource dictionaries.

src/Avalonia.Styling/Controls/ResourceDictionary.cs

@@ -12,7 +12,7 @@ namespace Avalonia.Controls
     /// <summary>
     /// An indexed dictionary of resources.
     /// </summary>
-    public class ResourceDictionary : AvaloniaDictionary<string, object>, IResourceDictionary
+    public class ResourceDictionary : AvaloniaDictionary<object, object>, IResourceDictionary
     {
         private AvaloniaList<IResourceProvider> _mergedDictionaries;