More convenient QAIM API for QML

QAbstracItemModel has already a couple of methods to access data (data, index, parent,...). Those are exposed as Q_INVOKABLEs, which helps with QML integration.

However, roleNames() is not exposed to the meta-object system, which makes accessing a specific role of the model from QML rather inconvenient.

It might also be worth to add some further convenience API to either QALM or at least to QML's ListModel which does not require a QModelIndex in the accessor, but only the row index as an integer and the role.