Allow users to set the location of reference plane grips for a family

[elmorei]

Often with complex families, the grips for the reference plans will overlap or end up far from the part of the model associated with it.  It would greatly increase the ability to create usable, adjustable families if the users could set the grip location of a plane the same way that the flip control can be set..

[stanford_ri]

The ability to hide certain grips would also be helpful.