fix #21704: imported chords do not render

Recent changes to default style chordlist handling resulted in imported
scores never loading a chord list at all, causing them to render
incorrectly (no root or bass).  This is a proposed fix for that specific
issue that also addresses other potential problems in chordlist handling
(honoring the distinction between a truly empty chordlist and one that
merely contains no chord id's but does contain sym & token definitions).
update biab test reference
regressions: fix grace note playback
added light theme, removed native theme
fix make install
add Element widget to Image inspector
Merge pull request #394 from MarcSabatella/21704-chordlist-import

load chordlist if necessary before importing score (MusicXML, etc)
fix potential crash when reading a linked staff
regressions: fix slur playback