Re: [webcomponents] Per-type ready event for Custom Elements

WebComponentsReady is an event in the Polymer stack that fires when all
known registered custom elements are ready for interaction - X-Tag has
similar event (now just a hook into the polyfill's) called
DOMComponentsLoaded. Being notified that all elements in source are ready
for interaction is essential at any point in the page lifecycle when a new
element is defined.

This is not an X-Tag issue, it's a Web Components issue - one we need to
address.