Setting a non-QAIM model in a header model will result in incorrect orientation

E.g. a HorizontalHeaderView will lay out its items vertically. This can be seen by modifying the tests/manual/headerview example by setting 10 as the model (and perhaps removing the syncView binding).

Another issue we need to account for if we're going to support this use case is the model roles in the QML files. We'll need to copy what ComboBox has:

        text: control.textRole ? (Array.isArray(control.model) ? modelData[control.textRole] : model[control.textRole]) : modelData

Until we support those other models, we also need to document that only QAIM-based models are currently supported.