UpdatesPage: Fix invisible delegates taking up space and causing a ghost section header

First,

Port from custom QSortFilterProxyModel subclass to KItemModels

then,

Fix invisible delegates taking up space and causing a ghost section header

In Qt 6 ListView allocates space for invisible delegates as well (unlike Qt 5), so now the issue has surfaced tenfold.

Add a new model role to filter over, so we don't have to mess with questionably efficient row filter callbacks.