fix occassional crash caused by needlessly delayed signals (bko#363224)

As pointed out by the Valgrind trace in #363224, delaying the signal causes
the EffectWindow* argument to become invalid before the connected slot is
called (this is because Qt discards only delayed signal->slot calls where
the receiver gets deleted meanwhile, not the sender and definitely not
a random argument.
If the supposed glitches really happen, they should get fixed correctly,
and for all cases (I doubt only desktop number would be involved but
not e.g. shaded or minimized states).

https://phabricator.kde.org/D5164

CCBUG: 363224