Add feature to drag end on all selected clips with [ E ].

Probably need two variations of this:

1. all ends drag by the same amount, regardless of where they are -  this probably implies selecting the beginning or ending border for the drags based on the cursor position relative to the current clip.
2. ends drag only as the cursor meets their timeline position
(i.e. all end-drags are done only if crossed by cursor movement -
although i suppose that can be simulated by a split then delete). Perhaps this could drag whichever end of the selected clips is nearer the cursor position - some would be beginning drags, while others might be end drags. Useful or confusing? I think we could make our selection such that it is potentially useful.