Our extension prompt code is a horrible mash-up of various bits of Firefox code, and it needs updating. I'm going to try to copy over a bunch of Firefox tests, too.

This brings our code up-to-date with mozilla-central's ExtensionsUI.jsm and browser-addons.js. I've pulled apart the implementations of gXPInstallObserver and ExtensionsUI which were combined previously due to a lot of overlap. This turned out to be a mistake from a maintainability point of view.

Since in this patch I'm mostly trying to achieve parity with the Firefox code, I'll post the differences between ours and theirs which will be easier to read than this.

This is the difference between Firefox's gXPInstallObserver and ours.

This is the difference between Firefox's ExtensionsUI object and ours.

Part two: this keeps all of our appmenu buttons on the same page with regards to alert badges.

Part three: tests for all of the above. This is all copied from mozilla-central (browser/base/content/test/webextensions) with some changes to make it work in Thunderbird. Two tests were dropped because they test things that Thunderbird doesn't do.

Pushed by geoff@darktrojan.net:
https://hg.mozilla.org/comm-central/rev/808f7c2c6680
Update extensions prompts code to match Firefox. r=mkmelin
https://hg.mozilla.org/comm-central/rev/e3d50753c9d8
Make all appmenu toolbar buttons display alerts. r=mkmelin
https://hg.mozilla.org/comm-central/rev/a12bb1124869
Copy Firefox's extension prompt tests and adapt them to Thunderbird. r=mkmelin