[Xwt.WPF] Make sure the label of a composite button has the right For…

…egroundColor

If the ForegroundColor of the external button is changed by another mean,
we want anything that depends on it also following that update.

Xwt.WPF/Xwt.WPFBackend/ButtonBackend.cs

@@ -34,6 +34,7 @@
 using System.Text.RegularExpressions;
 using SWC = System.Windows.Controls;
 using Xwt.Backends;
+using System.Windows.Data;
 
 
 namespace Xwt.WPFBackend
@@ -120,6 +121,7 @@ public void SetContent (string label, bool useMnemonic, ImageDescription image,
 						labelCtrl.Content = accessText;
 					else
 						labelCtrl.Content = label;
+					labelCtrl.SetBinding (SWC.Label.ForegroundProperty, new Binding ("Foreground") { Source = Button });
 					grid.Children.Add (labelCtrl);
 				}
 				Button.Content = grid;