This is first step of refactoring task T77580.
Next step would be moving smaller portion of sequencer.c file back into BKE and renaming remaining files to get SEQ_ prefix.
Just a note, that there may be obvious stupidity in some cmakelists files, I mostly looked in existing files and copied what I assume should be in there, and fixed compile errors until I was able to compile sucessfully.
I am dfinitely unsure about chenge in windowmanager library (I was copying what compositor does):
set(LIB bf_editor_screen bf_sequencer )