Request all preview jobs for specific mount point to stop
and proceed with unmount request only when all such jobs reported stopping.
This change fixes unmounting when it is initiated from dolphin.
Unfortunately, if unmounting is initiated outside of dolphin,
for example from 'mounted devices' widget,
Solid only sends broadcast notification and doesn't wait for any reply,
thus unmounting may fail due to preview jobs not stopping in time.
But in such case repeated unmount request from user should succeed immediately.