Cleanup: split sequencer.c file

Move most functions into separate files.

File sequencer.c is reserved for alloc, free and duplicating functions.

There should be no functional changes.

source/blender/sequencer/intern/modifier.c

@@ -49,7 +49,6 @@
 #include "BLO_read_write.h"
 
 #include "render.h"
-#include "sequencer.h"
 
 static SequenceModifierTypeInfo *modifiersTypes[NUM_SEQUENCE_MODIFIER_TYPES];
 static bool modifierTypesInit = false;