Optionally attach modal dialogs

Add a preference /apps/mutter/general/attach_modal_dialogs. When true, instead
of having independent titlebars, modal dialogs appear attached to the titlebar
of the parent window and are moved together with the parent window.
https://bugzilla.gnome.org/show_bug.cgi?id=612726

src/core/frame.c

@@ -276,7 +276,7 @@ meta_frame_get_flags (MetaFrame *frame)
   if (META_WINDOW_ALLOWS_VERTICAL_RESIZE (frame->window))
     flags |= META_FRAME_ALLOWS_VERTICAL_RESIZE;
   
-  if (frame->window->has_focus)
+  if (meta_window_appears_focused (frame->window))
     flags |= META_FRAME_HAS_FOCUS;
 
   if (frame->window->shaded)