Ajax Requests panel - proof of concept

It's certainly not commit ready yet, but I wanted to open this pull request to facilitate feedback on this idea.

The code implements an "Ajax Requests" panel that lists out ajax requests that have occurred.  You can then click on one of the requests to replace your current debug toolbar with the toolbar rendered for that ajax request.

Would you consider integrating a panel like this?  If so, what sorts of changes would need to be made first?  I'll happily write some tests.  I'm also not very happy with storing the rendered toolbars in the session, so I'll probably have to come up with some other form of semi-persistent server-side storage.

Apologies if someone else has done something like this already - if they have I didn't find it.  I did find an old fork that did something like this (used the cache to store the most recent AJAX request and load that), but nothing that added a panel and allowed debugging multiple AJAX requests at once, which is probably a necessity in most cases.

Anyways, looking forward to your feedback.  Thanks!