This patch adds a function gtk_check_menu_item_set_show_toggle, which
can change the way check menu items and radio menu items look. The
default behaviour is unchanged from before applying this patch: Only
selected items and the item under the mouse show their toggle, with
the one under the mouse reversed. When this function is called with
the second parameter to TRUE (instead of FALSE, which gives the
default behaviour), all items will always show their toggle, and the
one under the mouse is not reversed.
 
(This was deemed desirable on the mailing list, as it seemed that
Motif was capable of doing both, and the default behaviour was judged
as confusing by some.)
 
It also patches testgtk.c a bit to show off this function.