Make semantic

[Stuart Langridge]

Possibly implement items as <li>s and allow size changes through dragging, rather than size changes through the item nodeName (h1-h6). This loses some semantic information, though. It is maybe possible to have an item be any node at all (for example, some may be <h1>-<h6> and some may be <li>s in a <ul>), but this is likely to be impossible without making people care about the underlying markup.