Content above/below invisible staff affects system distance

Related to #281024: Invisible staves inappropriately factored into note spacing but this time vertical rather than horizontal, and luckily, I think, easier to fix.

Score for three instruments. Add a note somewhere on third instrument way way way below staff. Now hide that instrument in Edit / Instruments. Result: extra space is reserved for it. That's because in System::minBottom(), we call System::lastVisibleStaff(), but that function considers only "hidden" staves (a la "Hide empty staves"), not invisible ones. Looking at the other places this function is used, I can't see any problem just fixing it to check both flags.