queue operations on busy disks

recallmenotrequested to merge
(removed):copy_queue_not_idle into master

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

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