spotrem: add volume knob gesture

[thyttan]

Can be tried at: https://thyttan.github.io/BangleApps/?id=spotrem

[bobrippling]

Very cool! Works well when I tested it. Do you think it's worth mentioning that the clockwise/anticlockwise gesture detection is stopped after a brief period of no-interaction?

Not for this PR, but if you wanted ideas on going further with this feature, it'd be cool to pull it out to a library and let other apps opt into the clockwise gesture, and during the clockwise gesture, display a circle on screen that follows the user's gesture. But obviously those are much bigger changes!

[thyttan]

Thanks :)

Do you think it's worth mentioning that the clockwise/anticlockwise gesture detection is stopped after a brief period of no-interaction?

Yes! Will add that.

[...] it'd be cool to pull it out to a library and let other apps opt into the clockwise gesture, and during the clockwise gesture, display a circle on screen that follows the user's gesture. But obviously those are much bigger changes!

Yes - that's where I started with this. But I switched to this focused implementation first to dial in the UX (pun intended).