[GTK/Paned] Fixed margin test failure

Xwt.Gtk/Xwt.GtkBackend/GtkEngine.cs

@@ -129,8 +129,8 @@ namespace Xwt.GtkBackend
 				((IGtkContainer)cont).ReplaceChild (oldWidget, newWidget);
 			}
 			else if (cont is Gtk.Notebook) {
-				Gtk.Notebook notebook = (Gtk.Notebook) cont;
-				Gtk.Notebook.NotebookChild nc = (Gtk.Notebook.NotebookChild) notebook[oldWidget];
+				Gtk.Notebook notebook = (Gtk.Notebook)cont;
+				Gtk.Notebook.NotebookChild nc = (Gtk.Notebook.NotebookChild)notebook [oldWidget];
 				var detachable = nc.Detachable;
 				var pos = nc.Position;
 				var reorderable = nc.Reorderable;
@@ -140,12 +140,27 @@ namespace Xwt.GtkBackend
 				notebook.Remove (oldWidget);
 				notebook.InsertPage (newWidget, label, pos);
 				
-				nc = (Gtk.Notebook.NotebookChild) notebook[newWidget];
+				nc = (Gtk.Notebook.NotebookChild)notebook [newWidget];
 				nc.Detachable = detachable;
 				nc.Reorderable = reorderable;
 				nc.TabExpand = tabExpand;
 				nc.TabFill = tabFill;
 			}
+			else if (cont is Gtk.Paned) {
+				var paned = (Gtk.Paned)cont;
+				var pc = (Gtk.Paned.PanedChild)paned[oldWidget];
+				var resize = pc.Resize;
+				var shrink = pc.Shrink;
+				var pos = paned.Position;
+				if (paned.Child1 == oldWidget) {
+					paned.Remove (oldWidget);
+					paned.Pack1 (newWidget, resize, shrink);
+				} else {
+					paned.Remove (oldWidget);
+					paned.Pack2 (newWidget, resize, shrink);
+				}
+				paned.Position = pos;
+			}
 			else if (cont is Gtk.Bin) {
 				((Gtk.Bin)cont).Remove (oldWidget);
 				((Gtk.Bin)cont).Child  = newWidget;