[Xwt] Adds Focus change handling

Xwt.Gtk/Xwt.GtkBackend/WindowBackend.cs

@@ -37,10 +37,14 @@ public class WindowBackend: WindowFrameBackend, IWindowBackend, IConstraintProvi
 		Gtk.Alignment alignment;
 		Gtk.MenuBar mainMenu;
 		Gtk.VBox mainBox;
-
+		public event Action<object> FocusChanged;
+
 		public override void Initialize ()
 		{
-			Window = new Gtk.Window ("");
+			Window = new Gtk.Window("");
+			Window.FocusChildSet += (s, e) => {
+				FocusChanged?.Invoke (e.Widget);
+			};
 			Window.Add (CreateMainLayout ());
 		}
 

Xwt.XamMac/Xwt.Mac/PopupWindowBackend.cs

@@ -53,6 +53,13 @@ public class PopupWindowBackend : NSPanel, IPopupWindowBackend, IUtilityWindowBa
 		bool sensitive = true;
 		bool isPopup = false;
 		PopupWindow.PopupType windowType;
+		public event Action<object> FocusChanged;
+
+		public override bool MakeFirstResponder(NSResponder aResponder)
+		{
+			FocusChanged?.Invoke (aResponder);
+			return base.MakeFirstResponder(aResponder);
+		}
 
 		public PopupWindowBackend ()
 		{

Xwt.XamMac/Xwt.Mac/WindowBackend.cs

@@ -476,6 +476,14 @@ public void SetMainMenu (IMenuBackend menu)
 
 		bool disposing, disposed;
 
+		public event Action<object> FocusChanged;
+
+		public override bool MakeFirstResponder(NSResponder aResponder)
+		{
+			FocusChanged?.Invoke(aResponder);
+			return base.MakeFirstResponder(aResponder);
+		}
+
 		protected override void Dispose(bool disposing)
 		{
 			if (!disposed && disposing)

Xwt/Xwt.Backends/IWindowBackend.cs

@@ -52,6 +52,8 @@ public interface IWindowBackend: IWindowFrameBackend, IChildPlacementHandler
 		/// Sets the minimum size of the window
 		/// </summary>
 		void SetMinSize (Size size);
+
+		event Action<object> FocusChanged;
 	}
 
 	public interface IWindowEventSink: IWindowFrameEventSink

Xwt/Xwt/WindowFrame.cs

@@ -124,6 +124,11 @@ public WindowFrame ()
 		{
 			if (!(base.BackendHost is WindowBackendHost))
 				throw new InvalidOperationException ("CreateBackendHost for WindowFrame did not return a WindowBackendHost instance");
+
+			var windowBackend = ((WindowBackendHost)base.BackendHost).Backend as IWindowBackend;
+			if (windowBackend != null) {
+				windowBackend.FocusChanged += OnFocusChanged;
+			}
 		}
 
 		public WindowFrame (string title): this ()
@@ -144,7 +149,12 @@ protected override void Dispose (bool disposing)
 		IWindowFrameBackend Backend {
 			get { return (IWindowFrameBackend) BackendHost.Backend; } 
 		}
-
+
+		public virtual void OnFocusChanged (object focused)
+		{
+
+		}
+
 		protected override BackendHost CreateBackendHost ()
 		{
 			return new WindowBackendHost ();