Remove the use_split_preview and use_result_cache parameters of
gimp_applicator_new(), and allow enabling/disabling the cache
(through gimp_applicator_set_cache()) and the preview crop (through
gimp_applicator_set_preview()) after construction.

Move the preview crop node after the result cache, and remove the
separate preview cache node.  This eliminates an extra cache
buffer, reducing the space consumed by filters, and speeds up split
preview, since the cached result now includes the output
compositing.