Bad barline spans with one-line staves after hiding / removing spanned staves

Ubuntu 14.04, GIT commit: 5c0b817

Reported here: https://musescore.org/en/node/68551. Investigation shows there are multiple aspects to this. The common element is a score for two staves - a five-line and one-line staff - in which bar lines are extended through them. You basically get bad results hiding or removing one of the two staves. The specifics depend on whether you are hiding or deleting a staff, but also on which staff is on top as well as on which staff is hidden or deleted

My forthcoming PR fixes all these cases and hopefully does not create any additional problems. The only glitch is that undo of making the bottom staff invisible (regardless of which of the two is the bottom staff) does not restore the original span of the bar line. But that was the case already, and I really didn't want to risk messing with the undo stack here.