Loops don't work with SimpleParallel BT composite node

Loop decorators don't work with SimpleParallel BT composite node. We need to either fix it, or if we remove support for it restrict users from putting these decorators on SimpleParallel's children.

The latter may be a huge pain in the neck since apart from just not displaying it in the list we need to also not allow drag&dropping a loop decorator from other nodes and re-parenting from a regular composite to a simple parallel