This adds click support for the select menu. Scrolling with the wheel
already worked.
If a custom OSC binds a button to a select.lua script-binding, this lets
users keep using the mouse to select an item.
While the OSC and the select menu are open at the same time, you can no
longer click the OSC's buttons. By using mp.add_key_binding instead of
add_forced_key_binding you could click both, but the console's binding
would be shadowed by MBTN_LEFT bindings in input.conf.