Rolling edit can now be triggered by selecting a timeline clip

and pressing Ctrl + Arrow key. This will start a rolling edit on the clip end that
is the closest to timeline cursor. Press again Ctrl + Left/Right Arrow to move edit point.
You currently have to press "Escape" or click "Close" in monitor to finish
operation, or kdenlive will be unstable if you try doing another operation
while in rolling mode.
Ref T1953