Spell input notes according to key

Previously, scherzo always preferred the sharp form of any note
pressed on the keyboard. Now, it uses the key to resolve any ambiguous
spellings of notes pressed.

This also means that chord spellings will now fit the key much more
nicely. (For example, in the key of F major, one will now see a Bb
major triad, rather than the very-odd-looking A# major triad).