Thunderbird conversations support

This is not quite pull request, but I wanted to poke you to take a look at my changes and discuss them.

I managed to make bugmail working with Thunderbird Conversations ( https://addons.mozilla.org/en-US/thunderbird/addon/gmail-conversation-view/ - excellent plugin which gives some gmail-like experience but reorganizes message pane noticeably ). The changes are noticeable (new events must be subscribed,  they provide different kind of information, interface must be modified differently) but should not spoil original code.

The code works, but so far works only for bugzilla, and is more a proof of concept than final solution (for example actual bugzilla-supporting code lies in overlay.js so far). I can work on moving it down to specific modules and refactoring them to reuse the code, but first I wanted to ask whether you are interested in supporting Conversations and open to accept noticeable code refactorings (for example APIs of tracker modules would need to change somewhat so they work properly with nsIMsgDBHdr and html-object data instead of message uri and document)

Best regards
Marcin