Accidental restructuring 1st attempt

[Rune Zedeler]

I have restructured the representation of the key signature so that each 
alteration is a smob of type Key_entry. Now the local key sig knows the 
measure-position where the notes associated with the accidentals ends.
I have changed the key engraver og acc engraver to use the new 
representation.
I have not made other changes yet - the acc engraver still uses the same 
algorithm and representation for rulez. I would like some comments on 
the representation of the key-sig.

I still need to add scheme functions to access the key_entries. For now 
there is only one scheme function - the one that creates them.

I could not figure out the right way to find out when a given not ends. 
Is the following correct?

      Duration *dur = unsmob_duration (note->get_property ("duration"));
      SCM smp = get_property ("measurePosition");
      Moment mp = robust_scm2moment (smp, Moment (0));
      Moment end_mp = mp.grace_part_ < Rational(0)
        ? Moment (mp.main_part_, mp.grace_part_+dur->get_length())
        : Moment (mp.main_part_+dur->get_length(), mp.grace_part_);

-Rune