core: Add meta_close_dialog_focus() vmethod

This is used to request key focus on the close dialog whenever
a window that is frozen would receive key focus. Also, ensure
that the dialog gets focus when first shown if the window was
meant to receive input.

https://bugzilla.gnome.org/show_bug.cgi?id=762083

src/core/window.c

@@ -4369,6 +4369,10 @@ meta_window_focus (MetaWindow  *window,
       clutter_stage_set_key_focus (stage, NULL);
     }
 
+  if (window->close_dialog &&
+      meta_close_dialog_is_visible (window->close_dialog))
+    meta_close_dialog_focus (window->close_dialog);
+
   if (window->wm_state_demands_attention)
     meta_window_unset_demands_attention(window);