Add N.C. entry to ChordNames context.

[n . puttock]

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_.

http://codereview.appspot.com/62076