Implementing a 'top term' meta field

This includes:
- a fix to the metabox api (oops)
- a new meta field for 'top_term' that allows the user to choose from all a post's categories, tags, and series which is the "top" one. This meta field is aware of state changes to categories and series during post editing and alters the dropdown accordingly (it's not state-aware of tags because those work very differently). 
- a new template tag, largo_top_term(), that displays this top term. Supports options similar to, and will fall back to, largo_categories_and_tags().