This allows any level of indentation for bullets. It does this by introducing
invalid HTML. In particular, you can make bullets like:
<ul>
<ul>
<li>foo</li>
</ul>
</ul>
1. Keeps all leaf nodes not just text nodes, so images etc. are not removed.
2. If the selection is not within a single block, it is expanded to the edges
of the blocks rather than splitting the blocks; this is unlikely to have
been what the user wanted.
3. More efficient tree traversal and manipulation; no duplication of nodes.
4. Records undo state before performing the action.
