Refuse to remove a picture when a modified local copy exists.

This check is done when the original picture is not available otherwise
we could loose editing done on the local copy.
Do not allow removing a film if containing non local copies.

And if the original picture is not accessible as we want to sync
the .xmp.
New routine to sync all .xmp for local copies.
Try to synch the .xmp for local copies if any at startup.

We want to make sure the .xmp are synchonized with the original files
as soon as they become accessible.