[AD] MSG_IDLE in grabber and menus

[Elias Pschernig <elias@xxxxxxxxxx>]

Just saw this:
http://www.allegro.cc/forums/view_thread.php?_id=324260#target

I think, both changes should be made. I'm not sure how it should be done
in the menu case. I think I remember, it was left as an open problem in
the last big GUI change. Maybe add a menu callback? Something like:

yield_callback = my_idle_function;

The menu code, as well as d_yield_proc, then could call that function.
Or maybe leave d_yield_proc - and add a new gui_menu_callback, which is
called as long as a menu is active.

-- 
Elias Pschernig <elias@xxxxxxxxxx>