**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
Move most functions into separate files.
File sequencer.c is reserved for alloc, free and duplicating functions.
There should be no functional changes.
