Works around bug 396102 at negligible performance cost, in a way
that won't break when KActivitiesStats gets fixed.
* I don't have time to fix KActivitiesStats.
* The maintainer of KActivitiesStats may not be available for a fix.
* There's an installed base of Plasma LTS which will not update KF5.
As a bonus, this fixes a crash when refresh() would have ever been
called during normal operation: The InvalidAppsFilterProxy and
GroupSortProxy models were creating circular parent-child relation-
ships by setting themselves as parents of their parent objects.