Tell gtk+ we really need the wayland backend

So that it doesn't choose the x11 backend when DISPLAY is set,
e.g. when running weston on x11 or when xwayland is available.

Require gtk+ 3.10 for this.

configure.ac

@@ -23,7 +23,7 @@ PKG_PROG_PKG_CONFIG()
 PKG_CHECK_MODULES([GTK], [
 	wayland-client >= 1.0.2
 	weston >= 1.3.0
-	gtk+-3.0 >= 3.7
+	gtk+-3.0 >= 3.10
 	gtk+-wayland-3.0
 	libgnome-menu-3.0
 	gnome-desktop-3.0

shell/gtk-shell.c

@@ -254,6 +254,8 @@ main(int argc, char *argv[])
 {
 	struct desktop *desktop;
 
+	gdk_set_allowed_backends ("wayland");
+
 	gtk_init(&argc, &argv);
 
 	desktop = malloc(sizeof *desktop);