attempt to expose class NoteHead

This is my attempt to expose NoteHead to the plugin framework.
NoteHead is needed because it contains the enum NoteHead::Group which is needed to change a note's head to slash for example. (Slash plugin would need that.)
The problem is, that a class needs a default constructor to be used with qmlRegisterType. NoteHead, however, is derived from Symbol which doesn't have one. So I added this to class NoteHead (line 85):
  NoteHead() : Symbol(0) {}
I don't know if that's the way it should be done, while it seems to work.