MusicXML: Convert all clefs, escape " in header fields, code simplification

MusicXML: Convert all clefs, escape " in header fields, code simplification

-) Extend the ClefChange class to convert all different clefs (G,C,F,
   percussion,tab) to the corresponding lilypond clef names. MusicXML
   also allows a "None" key, which lilypond does not :-(
-) In the header fields and the lyrics, escape all quotes by \". Only
   wrap the whole string in quotes if it is necessary (string contains
   numbers, spaces, quotes, or starts with a period or comma). This makes
   it much easier to edit the lyrics later on.
-) Several coding style issued pointed out by Han-Wen

Signed-off-by: Reinhold Kainhofer <reinhold@kainhofer.com>