VSE: Strip Transition Handles

Currently transitions in VSE (Blender's video editor) have usability issues, as transitions can't be on the same channel as the input strips, and there aren't dedicated transitions for audio strips. This project aims to address these main usability issues. The project adds handles to transitions (eg. crossfades) in the VSE, allowing users to move the transitions and change their length. When adding transitions, they get a default length. A right-click context menu - when clicking between strips - for adding transitions is added. Adjustable crossfades for sound strips are added.