Re: [w3c/webcomponents] Consider adding a childrenChangedCallback or something (#619)

Now in almost all implementations connectedCallback behave as connectedAndParsedChildrensCallback and child nodes are fully accessible. I think need left as is.
In Chrome/Canary if custom element define in markup - connectedCallback has empty child list, but if custom element added from js script - child nodes are fully accessible.

For example if need header/footer can write  this.innerHTML="Header ..."+this.innerHTML+"Footer...";  
With new specification this simple task will be impossible or need complicated code with childrenChangedCallback/MutationObservers. 
And if child content will be "children1 children2 children3 ... children_final" - need somehow determine moment when children_final added. If childrens has nested childrens - it can be very complicated task.

If custom component has nested element, and connectedCallback has empty child list - this event absolutelly useless for web developers. 


Custom elemets are very simple, but if read specification - very very complicated.

Must be so:
Firstly all dom structure must be created without any CEReactions(ecxeptions - adopt/upgrade), for custom elements need invoke only constructor or use HTMLUnknownElement.
CEReaction for element stars only after parent element finished connectedCallback event. Also first CEReaction for custom element must be connectedCallback(ecxeptions - adopt/upgrade). 


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