We need to move the result out of the BackgroundAction object before
posting the completion callback as there is a chance the
BackgroundAction instance gets freed before the event loop runs our
callback.
Fixes#7641
The worker thread used for BackgroundAction was going to sleep for
1 second at a time (when there was nothing to do.) This made using
background actions for anything interactive quite unresponsive since
you had to wait up to 1 second before it even started on your task.
We now use a simple Unix pipe to signal the worker thread that a new
work item is available.
This makes Assistant way more responsive when typing. :^)
