STILL TODO

- Add support for cite attribute of ins and del.
- Check cite attribute is valid URI.
- Retrieve metadata from cited URIs.
- Serialize copied quotations to legacy HTML, XHTML, BBCode, XBBCode and
  plain text.
- Sanitize copied quotations:
	* Check list items are in lists.
	* Strip comments.
	* Evaluate and strip script.
	* Remove some elements if empty: p, blockquote, q.
- Create a back-end format for quotations, allowing pasting in multiple
  formats.
- Add "Copy As Quotation" to Edit menu.

DONE
- Stopped showing "Copy as quotation" when not available.
- Added title to copied quotations based on document metadata.