Move most functions into separate files.
File sequencer.c is reserved for alloc, free and duplicating functions.
There should be no functional changes.
Move functions closely related to rendering images and proxies into
render.c and proxy.c files. render.h and proxy.h are created for
functions used internally.
There should be no functional changes.
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
When sequencer was moved from kernel, the DEFINES based on audaspace
were not moved along.
That means users had no way of creating new audio strips since
BKE_sequencer_add_sound_strip was ifdef'out
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
