clipproperties: Add a compositing expander

Pitivi allows users to manually keyframe the opacity of a clip to create fade-in and fade-out transitions.

However, this is relatively complex for basic use cases, and can be tedious to do on a large scale.

This commit adds a compositing expander to the clip properties panel, in which the user can easily set fade-in and fade-out durations for a clip.

Fixes #1472 (closed)