Create global variants for hardware

This MR provides a way to create variant widget which can be used with any hardware types.

It introduce:

• property variant, which is a lazy way to display something from a hardware object (if you design the server side object, you don't need to spend much time on the web side
• state variant, which displays a basic state from any hardware object

)

This was useful to merge MotorState and ShutterState together. It would be good to rework other hardware objects to provide such state, but it is not mandatory now, a basic state is also displayed for them.