wayland: implement transient hints for wayland clients

wl_shell supports a set_transient() map request that is equivalent
to setting WM_TRANSIENT_FOR in X11, so implement that.

https://bugzilla.gnome.org/show_bug.cgi?id=707401

src/core/window-private.h

@@ -127,6 +127,7 @@ struct _MetaWindow
   Window xtransient_for;
   Window xgroup_leader;
   Window xclient_leader;
+  MetaWindow *transient_for;
 
   /* Initial workspace property */
   int initial_workspace;  
@@ -727,4 +728,7 @@ void meta_window_set_gtk_dbus_properties  (MetaWindow *window,
                                            const char *application_object_path,
                                            const char *window_object_path);
 
+void meta_window_set_transient_for        (MetaWindow *window,
+                                           MetaWindow *parent);
+
 #endif