Move most functions into separate files.
File sequencer.c is reserved for alloc, free and duplicating functions.
There should be no functional changes.
Variables renaned:
- cfra -> timeline_frame
- nr -> frame index
- cfra_over -> overlap_frame
Function seq_give_stripelem_index was renamed to seq_give_frame_index.
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.
