Avoid freezes on SFTP/MTP mounts

Workaround for #207 when copying files from/to SFTP or MTP mounts.

Is there a reason why we can't use the [is_native()](https://developer.gnome.org/glibmm/stable/classGio_1_1File.html#aaa3b910bc9f240d4ebd0efc70979f163) method there? It seems to me that it would handle most edge cases properly, and both SFTP and MTP would have been handled by this.