Review interface for reviews

Patch reviews naturally consume a large space in an issue, and you often need to scroll to read all review comments in one review.

I not only want to improve the usability and interface for patch reviews here - I also want to shape the base for even more awesomeness in Drupal's development workflow. Details for the latter, I'll keep top secret for now, so let's just work on the interface. :)

Note: Do not keep any of the attached patches applied to your local user script. Only use them for testing in #631600: Manual Testing issue. Nowhere else, please.

What I have in mind is a vertical-tabs-alike interface, i.e. (auto-generated) labels on the left, and corresponding code + comment on the right.

The current patch rather implements a collapsible-fieldset-alike approach, but that has the negative effect of jumping labels. I.e. when you open the first comment, all other labels move down, and depending on the size of the first comment, they may even be moved outside of the visible viewport.

Vertical tabs, though, have the well-known limitation that you can only display one item at a time. Hence, you couldn't open more than one comment at the same time.

So we possibly need something different, in between vertical tabs and collapsible fieldsets.

+-----------------+-----------------------------------------------+
|                                                  Show/hide all  |
+-----------------+-----------------------------------------------+
|  Issue #1       | [code for issue #2]                           |
+-----------------+                                               |
|/ Issue #2 //////| Comment for issue #2.                         |
+-----------------+-----------------------------------------------+
|  Issue #3       | [code for issue #4]                           |
+-----------------+                                               |
|/ Issue #4 //////| Comment for issue #4.                         |
+-----------------+                                               |
|                 |                                               |
|                 |                                               |
|                 |                                               |
+-----------------+-----------------------------------------------+

/// is a background-color in ASCII ;)