Use buttons to start and pause timer

Instead of tapping twice to start/stop the timer, allow the user to use the bottom(long) button to start/stop it. Rotating the disc works fine for resetting it.

Changed the title from "Use buttons for Timer App" to "Core: use buttons to start and pause timer"

Thanks for your suggestion. The reason we decided to use capacitive touch was to keep it consistent across other apps (e.g. metronome). People there have also suggested to allow a physical button to trigger playback. I believe probably the best solution down the road would be in the next hardware generation to include a third physical button specifically to turn things on and off.