Add N.C. entry to ChordNames context.

Add N.C. entry to ChordNames context.
Adding a rest to the music passed to a ChordNames context will
now create an N.C. ChordName.  N.C. ChordNames behave just like
regular ChordNames with respect to chordChanges.

[Neil Puttock, 2009-05-10 23:17:15]

http://codereview.appspot.com/62076/diff/7/1008
File lily/chord-name-engraver.cc (right):

http://codereview.appspot.com/62076/diff/7/1008#newcode65
Line 65: SCM no_chord_markup = get_property ("noChordSymbol");
\set noChordSymbol = ##f will cause (harmless but annoying) errors here. You
could check whether the markup's valid using Text_interface::is_markup ().

http://codereview.appspot.com/62076/diff/7/1008#newcode70
Line 70: chord_name_->set_property ("begin-of-line-visible", SCM_BOOL_T);
If you just retrieved the markup instead and used the existing code below, you
wouldn't need all this duplication.

http://codereview.appspot.com/62076/diff/7/1008#newcode72
Line 72: rest_event_ = 0;
You don't need this, since you're clearing it in stop_translation_timestep ().

http://codereview.appspot.com/62076/diff/7/1008#newcode73
Line 73: return;
You'd normally only return from process_music () if there's an error, so it
would make sense to check for rests after checking for noteheads.  This would
allow you to use the same code below to create the grob, set the markup and
calculate last_chord_.

[Neil Puttock, 2009-05-13 21:36:40]

http://codereview.appspot.com/62076/diff/2001/1016
File lily/chord-name-engraver.cc (right):

http://codereview.appspot.com/62076/diff/2001/1016#newcode20
Line 20: #include "text-interface.hh"
Goes above #include "warn.hh"

http://codereview.appspot.com/62076/diff/2001/1016#newcode63
Line 63: SCM chord_changes = get_property("chordChanges");
Move this back to its former position?

http://codereview.appspot.com/62076/diff/2001/1016#newcode75
Line 75: markup = scm_from_locale_string("");
Better to return if no markup found.

http://codereview.appspot.com/62076/diff/2001/1016#newcode149
Line 149: ASSIGN_EVENT_ONCE(rest_event_ , ev);
ASSIGN_EVENT_ONCE (rest_event_, ev);

http://codereview.appspot.com/62076/diff/2001/1016#newcode150
Line 150: 
remove blank line