List item support

[cfraz89]

Implements support for list items. This is anything with css display: list-item, including naturally li's.
So far respects list-style-type:

• disc (naturally in an ul),
• decimal (naturally in an ol),
• lower-alpha,
• upper-alpha,
• circle,
• square.
• disclosure-open
• disclosure-closed

Handles nested lists.

Note that it breaks out inline_layout_data seperate from the NodeSpecificData enum, as at one point the list item data was implemented as NodeSpecificData too and needed to overlap with inline layout data, until i realised nodes such as input could also be display: list-item and so list-items also needed to be broken out. While inline_layout_data could be moved back into NodeSpecificData, I think it makes sense to leave it out, as afaik layout is independent of the tag type, and there will probably still be overlaps.

Whats missing so far:

• Respecting li::marker, eg content, color.
• Other list-style-types
• Doesn't layout the same as browsers when <li> aren't nested in <ul> or <ol>, eg if they're nested in <div> instead. I can't quite reconcile how the browsers are behaving here, they compute the styling as list-style-position: outside but the layout seems to behave as if it's inside.

Demonstration has been added to text example.