[kcm/kwinrules] Fix first item position on property list

The first time the list in Add properties is shown, the position of the first
delegate is miscalculated as (0, 0) and gets hidden behind the section header.
It only repositions when the delegate is destroyed and re-created, for example, when
setting and deleting a filter text. (https://bugs.kde.org/show_bug.cgi?id=422289)

This works-around it by delaying the overlayModel filtering until the list is first
shown, which makes the delegate position correctly.

BUG: 421583
FIXED-IN: 5.19