Hide controls while metronome is running

Option for when play is started controls disappear and whole screen shows count. Only when pause is selected will onscreen controls re-appear.

Changed the title from "Display option" to "App: hide controls while metronome is running"

I assume this would have the advantage of having a better user interface while the metronome is running to show various indicators more clearly, and/or to avoid accidental changes/inputs. I have been thinking about that for a while already! It's similar to what running apps do while you're running, for example.