New Lucid menus

[Stefan Monnier <monnier@iro.umontreal.ca>]

I really like the antialiased fonts in the Lucid menus, but the code
still has some problems:
- emacs -Q followed by C-mouse-3 gets us straight to a segfault because
  of a "childs[0] = 0;" where childs has just been initialised to NULL.
  But even after removing this assignment I still get some assertion
  failure:

   % emacs -Q
   C-down-mouse-2
   ...move mouse down through the menu (making the submenus show up) and
   then back up to just above the menu title...
   ...release the mouse-2 button outside the menu

  I then get an assertion failure at insdel.c:2046 because
  "XWINDOW (selected_window)->buffer" doesn't hold a buffer apparently :-(

- when I move the mouse through the menubar's submenus, they flicker like
  crazy: they seem to be constantly redrawn.


        Stefan