Decouple file transfers from GUI

One thing I have noticed multiple times already is that when the GUI hangs for some reason (usually low network bandwidth while copying from / to network shares and simultaneously browsing that network share in another files instance), it might hang up totally so I have to kill the files app.

However, that also aborts the running file transfer while that file transfer is not complete. So to reduce the consequences of the files app hanging up, the actual file transfer job should be started in a different process