Fix: clock losing time.

[mycroftw]

I was concerned that timer precision was such that going down 1 second every second click would lose significant time. Turns out it's about 1 second every two minutes. Unacceptable - for a clock.

• Replace self.clock with self.round_end, calculated whenever needed;
• Added a TimeSpan "time left when pause button clicked", so that when we come off pause, we can reset round_end;
• Sped up the "second timer" to fire 4/second, and instead of subtracting a second, calculate the time left and display it;
• Moved the game start a bit around so that time between "set conditions" and "start clock" doesn't tick down the timer;
• Ensured the timers are stopped when the program closes (documentation elsewhere stated that they could remain as zombie processes if not explicitly stopped);
• Got rid of some unnecessary debugging.

Fixes #18

[mycroftw]

Need to test more, but initial checks show it working.