Feature/metronome-shortcut

This pull request is focused on the metronome functionality. In particular, the result of this pull request should:

- Add the ability to start/stop the metronome by pressing **?** or **/** key (keyboard shortcuts must be enabled).
- Update the *info* modal dialog so that the shortcut is displayed in the legend (Jeff Martini).
- Fix issue with the metronome in Safari (only plays once). Ensure it works in Chrome, Safari, and FF.
- Refactor metronome views/models.

Note: In order to implement this functionality, I refactored the metronome view into a separate view component and shifted some responsibilities around to maintain a better separation of concerns. As a result, this pull request is a bit larger than it strictly need to be if I was just implementing the shortcut functionality, however I think the refactoring was necessary to make things easier to modify going forward.