This value was meant to be used for keeping images that are slowest to
render in cache. Method of measurement was flawed, because it doesn't
take UI overhead into consideration.
Cache panel is to be removed because users should not have to tweak
settings like this. It is not useful for development either, therefore
it is removed completely.
Move most functions into separate files.
File sequencer.c is reserved for alloc, free and duplicating functions.
There should be no functional changes.
