This should make VSE code more readable and easier to understand from an
outside perspective.
The name was chosen to be `channel` rather than `channel_index` to keep
things short and concise -- it should be clear based on the context
whether we are talking about the strip's channel index (singular case,
`Strip::channel` or `SeqTimelineChannel::index`) vs. the channel list
(plural case, e.g. `Editing::channels`).
Pull Request: https://projects.blender.org/blender/blender/pulls/138919
35cab63eaf