metapage <-> metaframe initialization via window.postMessage
This process can only start when the metapage consumer adds the child metaframe.iframe elements to the DOM (iframes elements only start loading when added to the DOM).
Important note: both the metapage parent and the metaframe children do not start listening to postMessage events until the page they are running in has finished loading. If activity starts before this, all sorts of errors abound.
Adding the iframes to the DOM is outside the control of this library.
