WYTIWYG chord symbols

This pull request enables "what you type is what you get" for chord symbols as the default mode of MuseScore.  The general style dialog now offers a choice between "Standard" and "Jazz" chord styles, with a third option "Custom" that allows you to select a specific chord description file as before if you really want.  This pull request includes the necessary new chord description files to support Standard and Jazz styles.  These files will not define a chord id list at all, so all chords will be "unrecognized".  But they define a set of token-rendering rules that allow these unrecognized chords to be rendered just as well as with any of the existing chord description files.  The bottom line is that chords will now be rendered as typed, but with all the nice formatting that was previously available only for the specific chords recognized by the chord description file.  Now, just about anything can be rendered just as nicely.

This pull request relies on previous code already merged to handle unrecognized chords well - allowing them to be transposed and rendered well.  This pull request also adds support MusicXML export, so at this point, unrecognized chords can now do everything users would expect of any chord.  MusicXML import is also added.  The changes to support MusicXML actually form the bulk of the new / modified code here.  This pull request supercedes various other MusicXML-related pull requests I had made previously (most recently #381 )

Here is a link to an updated proposal / design document describing what is going on in more detail:

https://docs.google.com/document/d/1Q7LXHDSkQb70xmcyquFSiSv_lGbOXMDwzn_24-n7qJw/edit?usp=sharing

This pull request creates a slight conflict with #371 supporting capo chords.  If that request is merged before this one, a small change will be needed in harmony.cpp to resolve some overlapping code.