Add a "Toggle" option to the button mode

In playback mode, it is possible to write a scene to given fader. It would be extremely convenient to be able to enable a button mode that permits the button under the fader to initiate the playback (or playback sequence). Right now Flash, Solo, Solo Change, and Move/GO are the only options. Being able to layer the playback via the buttons would simply operations for a number of different venues.