Add qml import for KItemModels
QML dependency is optional and all original classes are kept away from
any new dependency.
The plugin extends KConcatenateRowsProxyModel via extended types to
expose sources as QQmlListProperty in a declarative way.
Intention is to expose some of the models we have here, but also the
QSortFilterProxy wrappers we have currently incorrectly in
plasma-framework and some common models used across plasma workspace.