[WPF] Proxy Label SetFocus call to the underlying TextBlock so that i… (#868)

* [WPF] Proxy Label SetFocus call to the underlying TextBlock so that it works

Xwt.WPF/Xwt.WPFBackend/LabelBackend.cs

@@ -35,6 +35,7 @@ using SWD = System.Windows.Documents;
 
 using Xwt.Backends;
 using System.Windows.Automation.Peers;
+using System.Windows.Threading;
 
 namespace Xwt.WPFBackend
 {
@@ -82,6 +83,29 @@ namespace Xwt.WPFBackend
 			}
 		}
 
+		void FocusOnUIThread ()
+		{
+			// Using Render (7) priority here instead of default Normal (9) so that
+			// the component has some time to initialize and get ready to receive the focus
+			Widget.Dispatcher.BeginInvoke ((Action) (() => {
+				((WpfLabel) Widget).TextBlock.Focus ();
+			}), DispatcherPriority.Render);
+		}
+
+		public new void SetFocus ()
+		{
+			if (Widget.IsLoaded)
+				FocusOnUIThread ();
+			else
+				Widget.Loaded += DeferredFocus;
+		}
+
+		void DeferredFocus (object sender, RoutedEventArgs e)
+		{
+			Widget.Loaded -= DeferredFocus;
+			FocusOnUIThread ();
+		}
+
 		public bool Selectable { get; set; } // TODO: this is only supported on Win10 with UWP?
 
 		public void SetFormattedText (FormattedText text)

Xwt.WPF/Xwt.WPFBackend/WidgetBackend.cs

@@ -270,17 +270,18 @@ namespace Xwt.WPFBackend
 
 		void FocusOnUIThread ()
 		{
-			Widget.Dispatcher.BeginInvoke ((Action) (() =>
+			// Using Render (7) priority here instead of default Normal (9) so that
-			{
+			// the component has some time to initialize and get ready to receive the focus
+			Widget.Dispatcher.BeginInvoke ((Action) (() => {
 				Widget.Focus ();
 			}), SW.Threading.DispatcherPriority.Render);
 		}
 
 		public void SetFocus ()
 		{
-			if (Widget.IsLoaded) {
+			if (Widget.IsLoaded)
 				FocusOnUIThread ();
-			} else
+			else
 				Widget.Loaded += DeferredFocus;
 		}