frames: Give Havoc and Owen a heart attack

Break down the beautiful core/ui abstraction barrier by inserting
a pointer to MetaWindow into a MetaUIFrame. I'm a scoundrel, I know.
We'll use this very soon to destroy meta_core_get.

src/core/frame.c

@@ -87,6 +87,7 @@ meta_window_ensure_frame (MetaWindow *window)
 
   frame->ui_frame = meta_ui_create_frame (window->screen->ui,
                                           window->display->xdisplay,
+                                          frame->window,
                                           visual,
                                           frame->rect.x,
                                           frame->rect.y,