Gaps: Guitar Ear Training

There are now shadows that limit the frets the degrees exercise shows as valid for a question. This forces you to learn multiple places to locate degrees at, since a degree can usually be found both a couple of frets lower and a couple of frets higher on the neck than the root note. ## Other improvements: - Choose the input device. Fixes a problem where some audio interfaces won't allow connecting a guitar cable in their 1st input and Gaps would leave no option to use the other channels of the interface, making the audio input impossible.