Blender Git Loki

VSE UX: Make Speed Effect strips more user friendly.

**Drawing Changes:**
- F-curve drawing for Stretch, Multiply, Length and Frame Number.
- Value drawing when no keyframes for Stretch, Length and Frame Numbers.

General view of the new drawing for each speed effect mode:
{F9796642, size=full}

Detail of the horizontal zero (blue) line in the new `Multiply` mode:
{F9798520, size=full}

Nice to have (but I don't know how):
- Auto adjusting of endframe when using Multiply or Boost.

Differential Revision: https://developer.blender.org/D6110