[Feature request] Add ability to stretch/compress/scale the overall duration of a selection

[levilindsey]

Right now, if I select a range of notes in a track, then select the edge of a note and drag it to the left, each individual note in the track is shortened. However, this leaves the notes with bigger gaps between them than they had before, it doesn't change the start time of any of the notes, and it doesn't change the overall duration of the selection.

I would like to be able to resize the overall collection of notes. E.g., if I had a sequence of four adjacent quarter notes, with no rests between them, I want to be able to select a range spanning all four notes, then drag the end note toward the left to resize each individual note to now be an eight note, but still with no rests between them, and the total duration of the sequence now being half the value it was before.