Shape missing in device class

[THuppke]

The Shape is currently located in the PlacementConfig of DeviceConfig and UnitConfig. Since it makes sense to define a shape for a class of devices it should be possible to define it in the DeviceClass and optionally overwrite it in the DeviceConfig. Not sure whether the shape(and placement) is necessary in the UnitConfig as it is device-bound. At least it should not be in DeviceConfig and UnitConfig.