libkcups/{PrinterModel,PPDModel}: Extend models for widget-to-QML migration

• PrinterModel: add a few needed roles, add a property
• PPDModel: expose a load method, remove superfluous property "count"

Loading the PPDModel (printer makes and models) is not heavy but it's slow and it blocks other CUPS calls. The existing apps load the model every time it's needed. For the KCM, we'll only load the model once and this will give the qml side some flexibility on when to do do that.