New: key.sig.showchange is set with CautionaryNaturalsInKeySignatures
New: Initial support for alternate note headstyles
Fixed: Grace note ID collisions

Previously it was possible to generate an ID collision with grace notes. This commit adds the horizontal offset to the grace note ID to further reduce the possibility of collisions.
Fixed: Sibelius allows verse on chord, which is not allowed in MEI.

For now, a warning will be generated when this is encountered.
New: Chord objects now have an ID attached to them.
Code cleanup
Fixed: Character Entities are now properly encoded