Mullein

Copperbox revision 564.

Some work on printing uncommon key signatures (e.g. signatures with both sharps and flats). However this work is not very good and incomplete - the implement has partial functions that throw errors (which is unsatisfactory as a general rule) and missing functionality. It might be better simply to provide a user hook so that someone wanting an unusual key signature can simply supply it as literal LilyPond snippet.