fix [some] issues when adding/removing output ports
add options to show/hide master and midi tracks;; hide inactive tracks
meterbridge: handle shift+scroll
update meterbridge style: peak-color + label-size
prepare meter configuration..

- rework metric/tick image cache invalidation
  - flush cache
  - send signal to queue redraws
  (TODO) selectively flush cache // cache per size, style
- add four common line up levels