Add a functional game pause button.

Adds a pause button that's only displayed when the game is running. Pausing the game displays a grey overlay over the screen. Tapping the overlay resumes the game. If game time is adjusted during game pause, the game is not resumed.
This start() function is called once the players are added, not when the game timer is actually started. Thus it should keep the onBreak status on. Also the game shouldn't be paused in the beginning anyway.