wayland: Don't round trip recursively during initialization

Instead use asynchronous round trips that is synchronized in the end of
the initialization. This makes it easier to track state, as we won't
dispatch arbitrary Wayland messages while processing globals.

https://bugzilla.gnome.org/show_bug.cgi?id=719819

gdk/wayland/gdkdisplay-wayland.h

@@ -72,6 +72,8 @@ struct _GdkWaylandDisplay
   struct wl_data_device_manager *data_device_manager;
   struct wl_subcompositor *subcompositor;
 
+  GList *async_roundtrips;
+
   struct wl_cursor_theme *scaled_cursor_themes[GDK_WAYLAND_THEME_SCALES_COUNT];
   gchar *cursor_theme_name;
   int cursor_theme_size;