Key-based cursor note movement and pitch2tpc()

Replace pitch2tpc2() and both variants of pitch2tpc() with a single
generic pitch2tpc(). Add PREFER_SHARPS, PREFER_NEAREST and PREFER_FLATS.
Update all places where those functions are called from.
Use intelligent key-aware stepping of pitch and tpc in Score::upDown().
Use KEY_C instead of 0 when calling pitch2tpc()
Remove code no longer used...
Update a couple of comments...
Merge pull request #214 from softins/7510-key-based-enharmonics

Key-based cursor note movement and pitch2tpc()