This fixes Virtual-keyboard velocity selection when scrolling
to numbers that are not explicitly in the menu-list.
(hopefully this does not introduce some recursive signals
for other ArdourDropdowns, fingers crossed).
This fixes an issue with scroll-wheel control which uses `get_active()`.
It work around an issue with gtkmm:
const MenuItem* get_active () const
void set_active (guint index)
and MenuList::activate_item() not emitting activate_item().