Manual adjustment to system divider (including making invisible) lost on save

It's actually lost immediately upon saving - no need to wait for save/restore. This worked when I first implemented it but I guess enough changed since then that it no longer does (and be careful not to have the divider selected when you save or it crashes - see #279064: Crash saving score with system divider selected

1) load attached score
2) drag a divider and/or mark it invisible
3) unselect it (unless you like crashes :-))
4) save

Result: it immediately reverts to its default state

I marked this P1 because it's loss of info and we don't want people spending a lot of time doing these adjustments then getting frustrating they are lost. But I wouldn't complain too bitterly if we merely disabled manual adjustments to system dividers for now (eg, by disabling selection). On the other hand, it worked once, might be a simple fix to get it working again, and it does seem likely to be useful.