We have introduced the signal trigger version of QtPromise in
d2a3732e. However, this is reverted in bdaba786 because it fails to
handle the case where the QtPromise is constructed in another thread than
the one where the executor runs. We introduce the signal trigger again
here and resolved the above problem by constructing it without a parent,
and then moving it into the thread of the executor.
This significantly reduces the CPU usage and CPU temperature on tusooa's computer.
Type: add