Moved annotation rendering from ce.Textnode into the new
ce.ContentBranchNode class. This allows us to render annotations that
span across multiple nodes.
* Add ce.ContentBranchNode, inheriting ce.BranchNode
* Make ce.{Paragraph,Heading,Preformatted}Node inherit ce.ContentBranchNode
* Made ce.ContentBranchNode render its child nodes with anntations,
using .getAnnotatedHtml() on the child nodes
* Put a default implementation for .getAnnotatedHtml() in ce.LeafNode
* Override this in ce.TextNode to do escaping and whitespace handling
* Removed rendering code from ce.TextNode (this.$ is now unused there)
* Removed ce.TextNode.onUpdate() and ce.BranchNode.clean(), now unneeded
* Have ce.BranchNode propagate update events from children, so
ce.ContentBranchNode can rerender when its children change
* Update tests, add test case for escaping of &<>'"
Change-Id: I4600e984b287c6ff9267f4281d2f09bab9e1ad95
