Vestax VCI-300: Beat sync/active changes and cleanup for 1.12

 - Changed mapping of the Auto Tempo / Sync button: Sync
   is now only triggered in combination with Shift, while
   pressing the button without the Shift key toggles
   quantize on/off. Before it was just to easy to
   accidentily trigger Sync by hitting the wrong button!
- Disable the beat active indicator LED, because it has
  not proven to be helpful.
- Some minor cleanups before adding new features for 1.12