Do ignore-filtered-siblings properly

Summary:
Turns out c8358c203f11 only worked accidentally. We can't do this
via filterModel->mapFromSource, because in a lambda connected to
its source model's rowsInserted signal, the proxy hasn't seen that
row yet.

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D15462