fix #40066: accidentals in chord symbols

Reverts default for German chords to use "b" and "#" by default rather than "es" and "is", for consistency with "Bb", which is still used rather than "B" to avoid possible ambiguity.  This was the consensus agreed upon in the issue thread and the forum discussion that led to it.  But it is now set up better and more clearly documented within the XML file to allow you to get a "pure German" mode with "B" instead of "Bb" and "es" / "is instead of "b" / "#".

Also fixes http://musescore.org/en/node/40091 - parser now recognizes "bb", "##", "x" as abbreviations for double flat/sharp.  Also "eses", "ses", and "isis" if in German mode.  And rendering is updated to use appropriate glyphs.