Stop button changes, button index 25 bugfix

BeatMatrix keeps references of all BeatButtons through buttonlist. Stop
button now asks each button to stop itself, so states return to STOPPED.
Fixed a bug where the last button would not play because it was getting
index = totalbuttons (button indices now begin at 0).
Newer buttons reuse indices, bugfix for restart

Buttons now reuse indices (0 through totalbuttons). The app currently
creates new buttons sometimes when restarted. Reusing indices seems to
fix a bug where the buttons would get indices higher than totalbuttons
and get out of bounds in the media player manager.