queue operations on busy disks
This resolves issue 1029.
On HDDs, simultaneous transfers are slow (heads constantly switching back and forth between files) and should be queued.
I applied bf5d28f6 of @alexxcons, adding THUNAR_PARALLEL_COPY_MODE_ONLY_LOCAL_IDLE_DEVICE, so now we can choose to queue transfers if a non-idle device is involved.
The credit for actually writing the changes goes to him, I merely adapted them to a current codebase.
Edited by recallmenot