MusicXML export of unrecognized chords

A chord description object is now generated for each (unique) unrecognized
chord, and MusicXML tags are generated during parsing for inclusion in the
chord description.  This allows unrecognized chords to be exported to
MusicXML using the same mechanisms as recognized chords.
MusicXML import of unrecognized chords

A new method is provided generate a chordname directly from a MusicXML
harmony object.  This chordname is then parsed and rendered just as for a
chordname typed directly into the score.
fixes

I have now verified that every single chord listed in chords.xml imports
and exports correctly.
initial commit
Merge branch 'chords-musicxml' into wytiwyg-chords
fixes
refactoring spanner, part I
refactoring grace note implementation
fix tests & handling of 1.X scores

also includes a do-over of the general style dialog changes, being more
careful not to accidentally alter other parts of file
better updating on chord style change
update Jazz Lead Sheet template for 2.0
improve handling of embedded chord lists

don't write on change from Standard to Jazz; write customized chords
even if generated; write other id-less chords
refactoring grace notes
generate HChord for unrecognized chords

also fixes write of voicing tag to score when custom chord list exists
updates
fix #11685: rename 'Create' menu to 'Insert', move View menu after Edit
add untranslatable strings
add untranslatable string
icon size in toolbar should respect preferences
single click expand in instrument selection dialog, fix tab order, search first