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Input position segment is no longer valid after time signature change.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.
Are we sure that icMeasure can't be null?