Emotion offset setter
The current way to change the offset of an axis is to set a new position. For a servo (DC) motor, whose position is always fluctuating, this operation is not deterministic. For instance, an exactly null offset is virtually impossible to set.
This patch proposes a controlled setting of the motor offset, a parameter that must be shared among all the motor clients.
The proposal uses the offset property setter in order to not change the current API