feat(view): add support for instantiation of nested views.

Includes shadowDom creation and integration tests for nested components.
Turns out this is infeasable, right now. The elementInjectors contain the view that contains the elements those injectors are in. There are many pointers going up, but currently no pointers going down the view tree (except in dirty-checking).

I kept childViews for now, but added comment to revisit, when the API starts to shape up better. I rather not keep ```addChild``` in the long run.