Two-Up View Redux

Building on what @Moistly started in #2395 (to fix #1475). Note that the following things might still be an issue:

 - What happens when you use the "bookmark" option in toolbar? Does it capture the two-page? I don't see that happening anywhere in the JS.
 - Why are the `<menuitem>`s that are acting like radiobuttons looking like checkboxes?
 - Should we have a separator between the two menuitem categories?
 - Are there getting to be too many items in the context menu? Mine is getting a little large...
![Untitled](https://f.cloud.github.com/assets/2175324/120928/da497c08-6d51-11e2-86aa-25d30d3fb232.png)