Add messages for the local copies and create a cancellable job.

This makes the process more user-friendly. Without indicator it was
not possible to know the progress and on slow drive the copy can take
some time.
Add tag darktable|local-copy to pictures with local copies.

This make it possible to select all pictures with local copies.
Update French translation.
Merge branch 'improve-local-copies'

* improve-local-copies:
  Update French translation.
  Add tag darktable|local-copy to pictures with local copies.
  Add messages for the local copies and create a cancellable job.