Blender Git Loki

VSE: Add operators to add and remove fades

Fades add:
Adds or updates a fade animation for either visual or audio strips.
Fade options:
- In, Out, In and Out create a fade animation of the given duration from
the start of the sequence, to the end of the sequence, or on boths sides
- From playhead: the fade animation goes from the start of sequences under the playhead to the playhead
- To playhead: the fade animation goes from the playhead to the end of sequences under the playhead
By default, the duration of the fade is 1 second.

Fades clear:
Removes fade animation from selected sequences.
Removes opacity or volume animation on selected sequences and resets the
property to a value of 1.0. Works on all types of sequences.

Author: gdquest

Reviewed By: ISS

Differential Revision: https://developer.blender.org/D5166