Expose position slots saved by G60 et.al. in Object Model

[oliof]

I could not find the slots used to store positions on pause/toolchange/g60 in the object model documentation at https://github.com/Duet3D/RepRapFirmware/wiki/Object-Model-Documentation -- I think it would be good to have those slots exposed there for more object model goodness.

[dc42]

@oliof they are already in state.restorePoints.

[oliof]

@dc42 great, I was searching for "slot" and "position". Even better!