This includes all the modes that don't need multi-object support: * SIMEDBONE_CHILDREN * SIMEDBONE_CHILDREN_IMMEDIATE * SIMEDBONE_SIBLINGS * SIMEDBONE_GROUP * SIMEDBONE_SHAPE Unlike the mesh select similar operator, in this case we take only the active bone in consideration, not all the selected ones. I decided not to change that, and keep this 2.79 behaviour. We could though if we want to.