Invalid SVG generation for filenames that have quotes embedded.

[Richard Shann]

In output-svg.scm this bit of code:

               (ly:format "<a style=\"color:inherit;\"
xlink:href=\"textedit://~a:~a:~a:~a\">\n"
                           ;; Backslashes are not valid
                           ;; file URI path separators.
                           (ly:string-percent-encode
                            (ly:string-substitute "\\" "/" file))
                           
                           (cadr location)
                           (caddr location)
                           (1+ (cadddr location)))

generates invalid xml if the file name file includes double quotes.
It should escape them thus:

                (ly:format "<a style=\"color:inherit;\"
xlink:href=\"textedit://~a:~a:~a:~a\">\n"
                           ;; Backslashes are not valid
                           ;; file URI path separators.
                           (ly:string-percent-encode
                             (ly:string-substitute "\"" "\\\"
                                (ly:string-substitute "\\" "/" file)))
                           
                           (cadr location)
                           (caddr location)
                           (1+ (cadddr location)))

This is a real problem for code (Denemo) which uses these links to do more
sophisticated things than just pass back a file name and location.