We don't need to reposition the menu every time its button is
allocated; we can just stick it in the right place when we pop it up
(which is guaranteed to not be during a layout cycle).

(This means that now we won't reposition the menu if the button
moves/resizes while the menu is already popped up, but it's not clear
that we'd want it to anyway, since that could easily result in the
user selecting the wrong item, etc.)

Also, we don't need to override the menu's width any more, so remove
that.