* MainWindow.cs: Add StartPosition and new menus

* Windows.cs: comment out broken code

* CommandBackend.cs: rename commands that do not match between Xwt and
  Gtk and ignore commands that do not have a Gtk StockProperty

* WindowBackend.cs: dummy implementation of SetServicesMenu and
  SetWindowMenu

* CommandBackend.cs: added commands

* CommandBackend.cs: add new commands

* MessageDialog.cs: rename GlobalCommand to StockCommand

* StockCommand.cs: add commands