Intern definitions are moved to sequencer/intern/sequencer.h
BKE_sequencer.h was also cleaned up a bit to make sure that functions
and structs are in correct category.
Reviewed By: sergey
Differential Revision: https://developer.blender.org/D9259
This is first step of refactoring task T77580.
Next step will be breaking up files into smaller ones.
Reviewed By: sergey, brecht
Differential Revision: https://developer.blender.org/D8492
