Review Mozilla 1891862: [meta] Unified Search Button

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1891862

Seems to be some new kind of UI for the search bar. I imagine this covers the new search popup that shows when clicked, and the search engine switcher.

Main reason to review this is to see how this integrates with about:torconnect. I.e. what happens if the user searches before we are bootstrapped. I imagine it will just redirect through about:torconnect like it does in 14.0, but maybe UX would want to use this as an opportunity to change the URL bar behaviour prior to bootstrapping.