Queries allow a directive to inject a live list of directives of a given
type from its LightDom. The injected list is Iterable (in JS and Dart).
It will be Observable when Observables are support in JS, for now it
maintains a simple list of onChange callbacks API.
To support queries, element injectors now maintain a list of
child injectors in the correct DOM order (dynamically updated by
viewports).
For performance reasons we allow only 3 active queries in an injector
subtree. The feature adds no overhead to the application when not
used. Queries walk the injector tree only during dynamic view
addition/removal as triggered by viewport directives.
Syncs changes between viewContainer on the render and logic sides.
Closes#792
