Set GDK backend to x11 explicitly

While this does not fix all issues when running on Wayland,
we at least get no hang when using GtkSelection and also can
get some basic drawing after the app was activated via the tray
icon. No keyboard shortcuts though.

re #27

src/gromit-mpx.c

@@ -1281,6 +1281,11 @@ int main (int argc, char **argv)
 {
   GromitData *data;
 
+  /*
+      we run okay under XWayland, but not native Wayland
+  */
+  gdk_set_allowed_backends ("x11");
+
   gtk_init (&argc, &argv);
   data = g_malloc0(sizeof (GromitData));