Redesign timer UI to show progress when clicking the play button

[mrreubenjones]

The "milliseconds" count is unnecessary, and could reduce performance. Find another way to show an instant visual response when clicking the play button.

• Remove "milliseconds" counter from timer
• Toggle play button with a stop (?) button, or something similar
• Use an animation to show how much time is remaining?

[mrreubenjones]

While I'm in there, I'm removing pause functionality, as discussed. (Pausing a pomodoro doesn't make sense, as the pomodoro is meant to be a chunk of focused time. If you pause the pomodoro, you may as well just start a new one, when you're ready.)