Always ensure setDestroyed propagates to all children applets
Some applets (systemtray, groupingplasmoids) can be parent of a Containment, in order to have nested containments (since the actual systray applet is a containment that instantiates other applets)
But also in that case we need to make sure that the destroyed property properly propagates to the children in order to not leak inner containments when they are deleted
(cherry picked from commit ff624952)
2f97b1de Always ensure setDestroyed propagates to all children applets
Co-authored-by: Marco Martin notmart@gmail.com