Redesign the time control selection screen
This MR fixes #23 (closed) with a major redesign of the time control selection screen.
- Custom time control entry is no longer hidden behind a popover, but is now front and center
- Preset buttons just set the values of the custom entry, rather than starting the game
- There's now a single, obvious button that starts the game
- Styling of the controls is improved, using the boxed list pattern
- The entire view is wrapped in a scrolled window for landscape support on phones