Cleanup and Documentation

Modified:
* A few changes were made to `MidiPlayer` that do not affect existing code.

Deleted:
* `Transport` (unused)
* `Frame` in GUI project (unused)
* `StringFret` class (unused)

Documentation added
* `Frame`
* `MidiPlayer`
* `Tablature` (except for the `parse` method, which will be handled by #18)