Change API to filter music fonts in text-interface

Change API to filter music fonts in text-interface

Currently, some grobs using the text-interface require
the music fonts (Emmentaler and Aybabtu).  The new changes
for the SVG backend require special handling for these
cases, thus the API change.

Now the SVG backend will use "glyph-string" to output text
in the music fonts, and "utf-8-string" for all other text.