Add -dmusic-strings-to-paths and enable for SVG backend. (issue186054)

[joeneeman]

http://codereview.appspot.com/186054/diff/1/2
File lily/pango-font.cc (right):

http://codereview.appspot.com/186054/diff/1/2#newcode384
lily/pango-font.cc:384: bool to_paths = get_program_option
("music-strings-to-paths");
It would be more consistent, I think, to use a global variable here.

http://codereview.appspot.com/186054/diff/1/2#newcode386
lily/pango-font.cc:386: if (has_utf8_string && ((to_paths &&
!music_string) || !to_paths))
If I understand this correctly, the SVG backend will now output feta
characters using utf-8-string by default (ie. unless the user manually
specifies -dmusic-strings-to-paths)?

http://codereview.appspot.com/186054