Refactor viewer-impl and keep it simple.

[lannka]

The viewer-impl.js is bloated. Any component that talks to the Search Viewer adds methods there. It doesn't just make the Viewer one-thousand line of code, it also had business logic scattered around.

We plan to bring back a slim viewer-impl.js, which should just provide basic methods like receiveMessage sendMessage getParam hasCapability.

We plan to move the methods to their own business components:

• history related methods => history-impl.js
• baseCid => cid-impl.js
• overlay, getPaddingTop => viewport-impl.js
• tick => performance-impl.js
• postDocumentLoaded => resources-impl.js
  etc.

A typical usage of the new viewer code will be like:

if (viewer.hasCapability('cid')) {
  viewer.sendMessage('cid', data).then(...);
}

[muxin]

• postScroll => viewport-impl.js
• postDocumentReady => resources-impl.js
• setFlushParams, flushTicks, prerenderComplete => performance-impl.js
• updateFragment, getFragment => stay in viewer-impl.js