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/wayland/meta-wayland-surface.h

@@ -75,6 +75,7 @@ typedef enum {
 typedef struct
 {
   MetaWaylandSurfaceType initial_type;
+  struct wl_resource *transient_for;
 
   char *title;
   char *wm_class;