fix #38766: bad input position after timesig change

Input position segment is no longer valid after time signature change.  I tried fixing this closer to where the change happens (Score::cmdAddTimeSig()), but while you can get the right segment there, the new measure isn't laid out until later and further upstream, in ScoreView::dropEvent().  And by moving the fix here, it turns out we solve a couple other problems.  For example, dropping a key signature can also cause a significant layout shift, and while the old segment is still valid so there is no corruption, the cursor position may *look* off.  This PR fixes that as well.
Not totally sure, but it seems pretty unlikely.  But I'll rewriteto handle that too.  And actually, it also occurs to me that this part of the code can be moved back to cmdAddTimeSig; only the mvoeCursor() needs to be here.  Then we aren't doing any of this unnecessarily.  Will update.