#427 Queue file copies

[Simon Descarpentries]

It would be usefull to enqeue new copy commands when a transfert is already running to avoid starving the disk bandwith. It's particularly usefull while copying a progressive selection of big files. Copies, done one by one, are still fast, and only one fails if the destination disk is abruptly disconnected.

The precious selection should not be lost in this case.