allow \button again for groupType \noteOnOffBut

[adcxyz]

In order to display the noteOn/noteOff/polytouch functions for pianoKeys and pads in a single GUI view, recent commits have set the elementTypes of simple on/off buttons using MIDI noteOnOff to type \pad as well. Now that this is working, regular \button views should be allowed again.
This will require a redirection equivalent to MPadViewUpRedirect for the button's separate on/off actions, while keeping gui-only toggle/latch mode usable.

[LFSaw]

sounds good to me 👍