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

This replaces https://github.com/musescore/MuseScore/pull/706; I needed to delete that branch in order to rebase.

Upon comments from lasconic, implementation is changed slightly.  The default is still for parts to inherit parent score style except for page layout settings, which come from defaultStyle.  But "Style for part" is now back to being monolithic - even if a specified style file is partial, a full style object is loaded globally in MScore, and this is used for parts. So parent score style is not inherited at all if any style for part is specified.

I also fixed a bug I discovered along the way where upon save & reload, parts were unintentionally inheriting changes made to parent score style after part generation.

Tests added for the latter bug as well as the default behavior (inherit parent score style except for page layout settings).