Blender Git Loki

VSE: Speed Effect layout updates

**Changes:**
- New enums correspond to 4 modes: `Stretch`, `Multiply`, `Frame Number` and `Length`.
- "`Multiply Factor`" has been removed;
- Value corresponding to "`use as speed`" enabled is now the value appended to the `Multiply` enum;
- Value corresponding to "`use as speed`" disabled is now the value appended to the `Frame Number` enum;
- Value corresponding to "`Scale to Length`" enabled is now the value appended to the `Length` enum;
- Except `Stretch` each mode has now its respective control values.

Differential Revision: https://developer.blender.org/D11856