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

I've given the matter of score & part styles a lot of thought, and feel the model I have implemented will work well:

- by default, parts inherit most settings from score, so custom font settings etc come through (just like 1.3)

- by default, the musescore default score settings are used for page layout, so that even if the score is horizontal and uses a small spatium, the parts will come out "normal" (an improvement over 1.3, since the first thing one usually had to do after generating parts was fix the page layout)

- if the "style for part" option is set, it takes precedence over both of the above, but partial styles are allowed, so the score style still serves as a fallback

With this in place, the need to have per-score "style for part" is greatly diminished: parts can inherit what they need from the score but still have some settings distinct.