* lily/grob-property.cc: add scm debugging hooks into
property modification
* lily/context-property.cc:
* lily/engraver.cc:
rename make_foo_from_properties to internal_make_foo and
move it from context-property.cc to a member function of
Engraver.
* lily/include/lily-guile-macros.hh:
overload ly_symbol2scm macro so that there is no more need
to use internal_foo