Implement a 'get children()' method in Action so extended classes can override it

[jbcpollak]

Currently we do something like this to fake it in a custom Action:

    constructor(child1, child2) {
        super();
        this.child1 = child1;
        this.child2 = child2;
        // TODO: this is a hack to render the tree
        // Implement getChildren() method in Blueshell
        this.children = [child1, child2];
    }

The onEvent(state, event) method doesn't use any of the existing child-handling support to decide which child to call.