Add -dmusic-strings-to-paths and enable for SVG backend.

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.

* Update comments.