Add isValidSetMember method to core API.

[pmeijer]

        /**
         * Checks if the node can be a member of the given set at the provided set-owner node. This does not take
         * cardinality rules into account.
         * @param {module:Core~Node} node - the node in question.
         * @param {module:Core~Node} setOwner - the owner of the set.
         * @param {string} name - the name of the set.
         *
         * @return {bool} The function returns true if according to the META rules, the given node is a valid
         * member of set of the given set-owner.
         *
         * @throws {CoreIllegalArgumentError} If some of the parameters don't match the input criteria.
         * @throws {CoreInternalError} If some internal error took place inside the core layers.
         */
        core.isValidSetMemberOf = function (node, setOwner, name) {...}

The same method is added to GMENode on the client.