Add -dmusic-strings-to-paths and enable for SVG backend.
* Using a -dmusic-strings-to-paths option generalizes the check
(lily/pango-font.cc) to see if the "utf-8-string" stencil
expression should be used for a backend.
* Enable the new option only if it's found in a list of supported
backends (scm/lily.scm). Currently, only the SVG backend
supports this. In the future, more backends might support this
option.