Implement framework for post-fix text (de)cresc spanners

Implement framework for post-fix text (de)cresc spanners
The main problem why \decr or \dim were prefix operators was that the
new dynamic engraver was reading only the properties of the Engraver
itself, so there was no way to create a CrescendoEvent and at the same
time specify the hairpin/text behavior.

This patch makes the New_dynamic_engraver also look at the properties
of the (Dec|C)recendoEvent and prefer a setting from there. If the event
does not have a property (which will be the case most of the time), the
Engraver's settings are used (i.e. same behavior as now).

With this change, e.g. \dim can be defined as
dim = #(make-music 'DecrescendoEvent 'span-direction START
        'decrescendoSpanner 'text 'decrescendoText "dim.")
and used as postfix like a4\dim without the need of any overrides
before the affected note.

[Neil Puttock, 2009-04-11 18:33:47]

I never realized this would be so simple, but it strikes me as a bit of a hack.

In your sample text dynamic spanners, there seems to be an element of redundancy
in the event properties; you could easily junk '(de)crescendoSpanner and
'(de)crescendoText, using 'text only to trigger the change, though I'm still not
sure I like this either:

+      SCM event_text = current_span_event_->get_property ("dynamic-text");
       SCM cresc_type = get_property ((start_type + "Spanner").c_str ());
 
-      if (cresc_type == ly_symbol2scm ("text"))
+      if (cresc_type == ly_symbol2scm ("text")
+	  || Text_interface::is_markup (event_text))
 	{
 	  current_spanner_
 	    = make_spanner ("DynamicTextSpanner",
 			    accepted_spanevents_drul_[START]->self_scm ());
 
-	  SCM text = get_property ((start_type + "Text").c_str ());
+	  SCM text = ((event_text != SCM_EOL)
+		      ? event_text
+		      : get_property ((start_type + "Text").c_str ()));
 	  if (Text_interface::is_markup (text))
 	    {
 	      current_spanner_->set_property ("text", text);
 	    }
 	}

http://codereview.appspot.com/39047/diff/1/2
File lily/new-dynamic-engraver.cc (right):

http://codereview.appspot.com/39047/diff/1/2#newcode76
Line 76: SCM spanner_type = evt->get_property (name);
This looks weird to me, since you're taking advantage of the fact that the event
properties you're setting look like context properties.  You'd need a method
which does the reverse of camel_case_to_lisp_identifier () so that the events
would be converted from e.g. crescendo-spanner -> crescendoSpanner.