Shift left/right command in note input create corrupt timing if durations do not match

Ubuntu 14.04, GIT commit: 648340f

Reported in https://musescore.org/en/node/86901

1) My First Score
2) Note input
5) 5 C 6 D 5 E (quarter, half, quarter)
6) Left Left (move cursor back to the half)
7) Shift+Left

Result: the first two notes are exchanged, yielding half quarter quarter, but the layout makes it clear something is wrong. The half note is allocated only enough space for a quarter, and the quarter is allocated space for a half.

Before:

After:

If you play the score, you hear quarter half quarter.

The object debugger reveals the segments are still at 0, 480, and 1440 - just as they were before the exchange. Looking at the code, I don't see any attempt to do anything like that; it seems the code is only going to work if the durations match. As short term fixes, we should probably either disallow the operation if the durations don't match, or exchange the durations.