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.
Write the local copy status into the database.

Also update the cache.
Display the local status into the metadata module.
Add a white circle on the image box in the ligttable area for local copies.

This is a simple marker to see what are the images having local copy
or not.