Allow for setting an "on selection change" action function for scroll and checkbox menus

[jwlodek]

Is your feature request related to a problem? Please describe.
Currently, the best solution to performing an action when a selection is changed in a menu is to unlock the draw loop, and run a function every second or so - this adds lag, and causes the function to run when it shouldn't.

Describe the solution you'd like
Add a method to the MenuImplementation for adding an optional "on selection change" event function that if set is fired with the new selected item as an argument every time the selection is changed.