layout updates

- Show app mode (timer/countdown) on top as a kind of sub-heading
- Emphasise main time display by showing it in the middle
- Make button groups full-width
- Highlight current app mode (using [disabled] style)
- Change stop icon to trashcan (best octicon for it I think, though
  no-smoking style cancel icon would be better)
Merge pull request #1 from mahemoff/gh-pages

layout updates