Because all uses of the function positioning menus anchored to a widget
were as callback argument to Gtk::Menu::popup() where the caller needed
to correctly bind arguments, this led to repeated and a bit obscure code.
Wrap the logic into an helper function that takes care of all that, and
update the callers.
Always give ArdourButton a chance to handle Press or Release events if you override them.
This allows ArdourButtons to do the "Pressed" animation
And it now correctly eats the event so button clicks don't select the channelstrip.
