#33 Add support for arbitrary length menus

[Christer Sandberg]

Menus that are longer then the longest distance to the
upper or lower edge of Allegro's window are clipped.
Currently the application has no proper way of
calculating the amount of space that is available in
order to avoid the problem.
As a result there is an implicit limit of the number of
items (if using a "single-menu") =
(allegro_widow_height - single_menu_height) /
menu_item_height
Only allegro_widow_height is available. Instead of
making the other measures available it would be better
that CGUI itself controls this, either by making the
last menu-item to a sub-menu-selection-item for more
menu-items, or by adding a browse-bar.