Prefer the Wayland backend

This does not affect X11 sessions, but has the advantage that
we get much more testing of the Wayland backend in Wayland
sessions.

gdk/gdkdisplaymanager.c

@@ -270,12 +270,12 @@ static GdkBackend gdk_backends[] = {
 #ifdef GDK_WINDOWING_WIN32
   { "win32",    _gdk_win32_display_open },
 #endif
-#ifdef GDK_WINDOWING_X11
-  { "x11",      _gdk_x11_display_open },
-#endif
 #ifdef GDK_WINDOWING_WAYLAND
   { "wayland",  _gdk_wayland_display_open },
 #endif
+#ifdef GDK_WINDOWING_X11
+  { "x11",      _gdk_x11_display_open },
+#endif
 #ifdef GDK_WINDOWING_MIR
   { "mir",  _gdk_mir_display_open },
 #endif