Allow choosing enharmonic when transposing

[barrettj12]

This song would probably be better rendered in Ab:

intro:
G# - D# - C# - C# - (D#)

verse:
G# - D# - C# - C# - (D#)
G# - D# - C# - C# - (?)
A#m - A#m - C# - G#
G# - D# - C# - C# - (D#)

chorus:
G# - D# - C# - C#   (x2)
G# - A# - C# - G#
G# - F# - C#

[barrettj12]

Probably a "switch enharmonic" button would be enough - generally there are gonna be at most two sensible choices. We don't need to allow the full infinite range of enharmonics.