This commit implements the actual UI changes. A follow-up commit adds the
tests for the changes. The CSS is a little bit awkard since it uses lots of
ID selectors rather than class selectors. I wanted to be able to write quick
selects, since it's selecting across the entire browser document. I feel
a little conflicted with the approach, as I would prefer to use classes in
general.
The panel.jsm.js file collects all of the UI handling changes rather than
having everything in menu-button.jsm.js, as the latter can get loaded
at startup. I'm not sure if it's completely worth the trouble of having
two files, as most of it should be pretty light.
This commit does not handle localization for the panel, as we should be moving
to Fluent. Rather than solve that here, I will follow-up with it in Bug 1599774.
Differential Revision: https://phabricator.services.mozilla.com/D62914
b3fb1bfd84