Fix #887: Use ly:string-percent-encode for textedit URIs.

Fix #887: Use ly:string-percent-encode for textedit URIs.

* Add an overloaded instance of String_convert::bin2hex optimized for
  converting single bytes to hex.

* Add a new callback, ly:string-percent-encode, to be used for percent
  escaping paths in textedit URIs.  This does the following:

  - Leave unreserved characters in textedit URIs unescaped.  This
    includes 0-9, A-Z, a-z, and three punctuation characters (hyphen,
    underscore, and full-stop).

  - Leave the forward slash (/) unescaped, since it is used as a path
    delimiter.

  - Escape all other characters.  Don't check for a null byte, since
    those likely won't sneak into a full pathname.

* Use the callback function in the PS backend.