fix #41786: barline span issues after edit / instrument

[MarcSabatella]

This fixes both https://musescore.org/en/node/41786 and https://musescore.org/en/node/68921, as well as similar issues where barline spans are invalid in some way after edit / instruments (see, for instance, https://musescore.org/en/node/68996).

The code for this comes from my PR #2122 , which is at this point still has issues I don't know how to resolve. However, this new PR stands on its own and is usable as is.

It comes with one small regression that to me is more than worth it. I added a TODO in the code about it, and I'll add a line comment there with more info.

[MarcSabatella]

So, here is the regression:

If you have a barline span of more than staff and it ends somewhere in the middle of the last staff, this customization will be lost if you make changes in Edit / Instruments. The barline will be reset to end at the bottom of the staff as it normally would. Workround: you can then restore the customized span and it will work fine.

I judge this to be not worth worrying about. Barlines spans of a single staff that end in the middle of a staff are common enough (eg, "tick" barlines), as are barlines spans of multiple staves that end above the last staff ("Mensurstrich"). I was able to make these cases work. I don't know of a use case for barline spans of multiple staves that end in the middle of a staff. I'm sure there is one, but if that is the only thing that gets messed up on Edit / Instruments, I would gladly accept that for now in order to fix the more common & more serious problems that this PR addresses.