Move Plasma's SortFilterProxyModel into KItemModel's QML plugin

Summary:
It exposes QSFPM in a usable way from QML, referring to roles by name
and exposing a way to perform JS callbacks as an advanced filter method.

Reviewers: broulik

Reviewed By: broulik

Subscribers: kmaterka, iasensio, ahmadsamir, broulik, ahiemstra, mart,
kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25326