Blender Git Loki

VSE: Refactor sorting functions

Recently `SEQ_sort()` function was split so functionality is provided
on per-seqbase basis. After discussion about this split, it turned out,
that per-seqbase operation is only that should be provided, because
RNA API functions need to be able to access arbitrary seqbase

Remove recently introduced function `seq_sort_seqbase` and change
`SEQ_sort` function to operate on seqbase.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D11297