Replaces the multiple arrays of `DomView`
by a single array with `DomElement`s.
Note: this commit does not show a performance regression
(tested against the tree benchmark locally).
Free embedded views are view instances that are created
logically in the same was as views of a ViewContainer,
but their dom nodes are not attached.
BREAKING CHANGE:
- `Renderer.detachFreeHostView` was renamed to
`Renderer.detachFreeView`
- `DomRenderer.getHostElement()` was generalized into
`DomRenderer.getRootNodes()`
When a template contains bound text nodes as root nodes,
we used to store the document fragment that we got from
cloning `template.content`. However, this fragment will be
empty as soon as the view gets attached. Now we store
`null` instead of the document fragment in this case.
Also groups the 3 cases in `_createView` so they are easier to
understand.
