Attach dialogs to windows with visual effects

Override the new mutter preference /apps/mutter/general/attach_modal_dialogs
to attach modal dialogs to their parent window. Animate the modal dialogs
expanding from the top of the parent window. Slowly dim the parent window
after the dialog comes up.
https://bugzilla.gnome.org/show_bug.cgi?id=612726

src/gnome-shell-plugin.c

@@ -154,6 +154,8 @@ static void
 gnome_shell_plugin_init (GnomeShellPlugin *shell_plugin)
 {
   g_setenv ("XDG_MENU_PREFIX", "gs-", TRUE);
+  meta_prefs_override_preference_location ("/apps/mutter/general/attach_modal_dialogs",
+                                           "/desktop/gnome/shell/windows/attach_modal_dialogs");
   meta_prefs_override_preference_location ("/apps/metacity/general/button_layout",
                                            "/desktop/gnome/shell/windows/button_layout");
 }