Called whenever a layout-node is created/re-used. Initializes
the node with the `insertSpec` if it has been defined and enabled
locking of the x/y translation so that the x/y position of the renderable
is immediately updated when the user scrolls the view.
function _initLayoutNode(node, spec) {
        if (!spec && this.options.flowOptions.insertSpec) {
            node.setSpec(this.options.flowOptions.insertSpec);
        }
    }