fix #24631: style not propagated to parts
fix #24732: update Style for part in prefs dialog
render chord symbols in part style
reinstate monolithic nature of style for part

style for part is back to being style object loaded in MScore and shared
by all scores, rather than being loaded into each part as they are
created.
partial style in part is relative to baseStyle

Fixed bug where, on read of score with parts, partial style in part was
being interpreted relative to score rather than to base style.  This was
leading changes in score style to inadvertently affects parts upon
reload.
add tests

basic style inheritance behavior tested, commented out placeholder for
test of "style for part" (need to figure out how to get test to honor
that settings)
Merge pull request #709 from MarcSabatella/24631-part-style

fix #24631 & #24732 - parts inherit score style by default, honor style for part option if set