Use a locally cached image if the original is not found.

This only put in place the circuitry to look for an image into the
local cache (default: ~/.cache/darktable) if not available on the
original location.

This simple command dt_image_full_path() has been enhanced to look
into the cache.
Add a way to set/reset an image to/from the cache.
Add local_copy field into images table.
Display the local status into the metadata module.