Let users pick the search engine from a dropdown in the urlbar

[VictorGugug]

When typing in the urlbar there's no way to choose which search engine to use from the panel itself. Firefox has the one-off buttons for this, but Zen hides them by default (zen.urlbar.hide-one-offs). The only options left are @ aliases or digging into settings.

I'd like a small dropdown in the urlbar that lists the engines the user has configured (Settings → Search) and lets them switch the active one right there. The list should follow the user's configured engines and be hidden when only one is set up.

Setting zen.urlbar.hide-one-offs to false already shows a row with the engines, so the existing one-offs could be reused as the base, either as they are or collapsed into a dropdown to save space in the compact urlbar.

If this gets the green light I can work on it myself, even if the current one-offs design ends up being good enough.

It would also be good to have this as a setting in Settings → Search, so users can turn it on or off themselves. Same for this kind of option in general: letting the user decide keeps full control of the browser in their hands.