Adds some features to the RunnerModel so that it can be used properly in the KRunner QML implementation I've been working on (see the testing section).

Also it simplifies the code a bit by moving from QAbstractItemModel -> QAbstractListModel.

use it in kde:scratch/apol/krunner-qml (proof of concept for a KRunner implemented in QtQuick).

here's a video, so that you know what's going on: http://www.proli.net/meu/netrunner/qmlrunner.ogv