CustomItemsProxyModel: Properly handle changes of the source model

Derive from QAbstractProxyModel instead of QSortFilterProxyModel
because the latter already handles changes of the source model, but not
as needed by CustomItemsProxyModel.

(cherry picked from commit c22f94eb1cf3486dd8b36401a8bdf8803355caee)

• GnuPG-bug-id: T7312