Cancel audio output when leaving screen

[noerw]

Thanks for the great app!

I noticed that played audio (notes as well as songs from the 'chord progressions in real songs' feature) keep playing, when i exit the exercise. This can be quite annoying.

[ShacharHarshuv]

Hey, I think this is resolved with this PR: #102, so the fix should be available in the next release.
However, even after this fix the last note played will sustain until it's over. I couldn't find a way to fix that yet. Songs from youtube will stop immediately.