Feature/no key

This pull request should add the ability to select "no key" on the UI.

- [x] Add "no key" entry to the *keyMap* configuration.
- [x] Add special case for mapping "no key" to a Vex.Flow key.
- [x] Review.

Note: I branched this off of *feature/metronome-shortcut* by accident so it's showing all of the changes from that branch as well. Once those changes are merged into the *develop* branch they should disappear from this pull request, just leaving the changes relevant to this PR.