PowerOn as standard Motor attribute

In #1693 (closed) we officially documented the Off state. However using it has a problem as it was identified at the end of this comment: #1693 (comment 861439374).

This issue is to discuss if we would accept the PowerOn standard motor attribute, which then could be implemented using SetAxisPar()/GetAxisPar() methods on the controller level as it is done for example for the velocity or base_rate - see docs.

First of all, I would like to ask you if you know any motion controller that does not allow to power on / power off its axes? Well even if there are some cases of such controllers we could accept this attribute because of the same reason as we have the Base_rate e.g. Icepap motion controller does not support this feature and we successfully use it with Sardana which provides such an attribute.

Having the PowerOn attribute would allow us to easily push the State attribute and unblock integration of !1723.