Add extra functions to transpose buttons

[pierstitus]

While fiddling a bit with the Striso board connected to the Duo some ideas popped up to make the Duo even more fun on itself.

I've added some functions to the transpose buttons to be able to play all chromatic notes, and to do absolute transpose. The first giving more tonal possibilities, the latter bringing more fun! Please give it a try.

Also now that all chromatic notes can be played I thought it made sense to give them colors.

Changed:

• Transpose on button release instead of press.

Added:

• Play notes a semitone sharp or flat while up resp. down button is pressed.
• Absolute transpose with the note keys when both up and down buttons are pressed. Center C# is zero transpose. This makes it possible to do more interesting transposing, and have an easy way to reset to the default.
• Add color to white button notes, now that they can be played on Duo.

[DavidMenting]

Thanks for this adaptation. We won't include this in the main branch as it changes the core functionality too much. It would be nice to have this included in an alternative firmware, though. I'm closing this pull request.