- Added PianoView: refactoring, move piano keys view creation
into a custom view. Split onClick handling between custom view
and controller (i.e. the main fragment)

- Added the full set of piano keys from B1 to D6. Added discreet
C4 label so it's easier for users to orient themselves.

- Fix images so we don't have ugly jagged borders. Regenerated
PNG24 versions.