Add a thumb informational element

[shaunix]

Thumbnails are useful for various extensions and specialty styles. Add a thumb element to info. Look at allowing multiple thumbs with type/role, just like title and desc.

[shaunix]

I've decided that the thumb element should live in Mallard core, not the UI extension. I've also revised the MEP to use the new proposal for common lookup rules for type-and-role data:

http://projectmallard.org/mep/mep0018

[shaunix]

And here's the thumb MEP, since I never posted in here:

http://projectmallard.org/mep/mep0004