effects: Disable showing desktop when starting any fullscreen effect

Fullscreen effects typically will show all the windows, when the effect
ends having the windows disappear doesn't really work.
From a user point of view any full screen effect is a new task, we want
to cancel any preivous task.

BUG: 487299

src/effect/effecthandler.cpp

@@ -419,6 +419,8 @@ void EffectsHandler::setActiveFullScreenEffect(Effect *e)
     Q_EMIT activeFullScreenEffectChanged();
     if (activeChanged) {
         Q_EMIT hasActiveFullScreenEffectChanged();
+
+        setShowingDesktop(false);
         workspace()->screenEdges()->checkBlocking();
     }
 }