Add IDataContext interface.

src/Avalonia.Base/IDataContext.cs

@@ -0,0 +1,10 @@
+namespace Avalonia
+{
+    public interface IDataContext : IAvaloniaObject
+    {
+        /// <summary>
+        /// Gets or sets the control's data context.
+        /// </summary>
+        object DataContext { get; set; }
+    }
+}

src/Avalonia.Styling/IStyledElement.cs

@@ -10,7 +10,8 @@ namespace Avalonia
         IStyleHost,
         ILogical,
         IResourceProvider,
-        IResourceNode
+        IResourceNode,
+        IDataContext
     {
         /// <summary>
         /// Occurs when the control has finished initialization.
@@ -27,11 +28,6 @@ namespace Avalonia
         /// </summary>
         new Classes Classes { get; set; }
 
-        /// <summary>
-        /// Gets or sets the control's data context.
-        /// </summary>
-        object DataContext { get; set; }
-
         /// <summary>
         /// Gets the control's logical parent.
         /// </summary>