Improve score widget

Our score widget isn't that pretty. There is a tiny lib which does the same thing but looks nice: https://github.com/DanielMartinus/Stepper-Touch We can just import it and implement correct handlers.

**Bonus feature:** increment/decrement in the Stepper-Touch lib are triggered by swipes to left or right. If we could mix additional "swipe and hold" logic into it, that would be awesome. So, when the user wants to increment more than by 1, we could either simply start incrementing further until the user releases the swipe, or we could display additional score slider:
- #29