Added Constraints Functionality

Add constraints, drag constraints, create thumbnails, scale thumbnails,
etc.
Still working on making the thumbnails visibly scale with the retimer
Worked on constraints retiming visuals

This isn't completely working, right now the constraints have the
correct connection between audio time/visual time, but the visuals don't
scale properly.

Also separated out the view updater from the retiming controller
Merge branch 'retimer' of github.com-jonathanwangmit:aehsu/pentimento