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

[n . puttock]

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.

http://codereview.appspot.com/39047