A new folder tree view for real

Seeing as how the first build candidates of Thunderbird 3.1 beta 2 are currently being spun, it is time for me to update from the 3.0 builds to 3.1 builds (I have a policy of switching to the next branch of Thunderbird as my primary at the time of the last beta). I decided to take this opportunity to work out issues in my folder categories extension for real.

I've decided to give up on supporting 3.0 (trying to support two different broken versions of code is not fun), so the oldest supported version is now listed as 3.1 beta 2. In reality, it would work with any nightly since the broken code was fixed (it's still not fully fixed now, but it's just a cosmetic issue). The result is an experimental addon on addons.mozilla.org. For those of you dying for screenshots, this page on mozdev should satiate you.