Make page up/down work again in scrolled menus

Make page up/down work again in scrolled menus

This was broken since before GTK+ 3.0, when we replaced
a use of requisition by allocation. Fix this by using the
requisition height, that is already cached by the menu code.

The math is not quite right here; if you page all the way
down a long menu, you end up on the second-to-last menuitem.
But at least, page up/down let you move up and down the menu
again.
https://bugzilla.gnome.org/show_bug.cgi?id=668931