New browser interface in the UI

Browser windows are what opens up when you click a presentation of an object.

I radically improved them today. Clicking a presentation inside a browser now re-uses the same window, and there is a history of previously-viewed objects. Also, a row of buttons across the top choose between multiple views of the object. This replaces the right-click command menu when clicking a presentation. Indeed, as strange as it sounds, I've removed support for popup menus from the UI altogether.

I also implemented help cross-referencing, analogous to the "calls out" (uses.) and "calls in" (usages.) features for words. You can now find out which articles link to the article you are reading now. This was made possible by implementing a small library to deal with directed graphs. I updated the word cross-referencing code to use this library too.

SCREENSHOT!