ContentEditableNode: Disable spellcheck when read-only
- Remove 'contentEditable' event support, which was triggered by BranchNode.disable(). The ability to disable a surface was removed when read-only support was added, so this code is no longer used.
- Emit a readOnly event from the surface, and have CE nodes listen to this and update their spellcheck property.
- Use setup/teardown events like in FocusableNode.
- Minor documentation fixes and cleanups.