Implementation bug for bug 1106032.

Interactive mockup: http://phlsa.github.io/fx-one-off-searches/

I'm unlikely to actually work on this soon. When I discussed this in person with Philipp in Portland, I remember the most obvious/pressing issues was that it was possible to end-up with 2 one-off buttons highlighted at the same time. This is not reflected in the bugs at all unfortunately, but I think this was the reason why we prioritized these bugs high. I fix this specific issue as part of the refactoring in bug 1124904. I think what remains here is: - unselecting the suggestions when the mouse moves out. I'm still not convinced we should do this, as it would be inconsistent with the awesomebar. - changing the blue highlight of the selected suggestion with a gray one when there's a one-off button selected. I think this is worth doing, but it's currently underspecified. The specific behavior that doesn't seem clear to me for example is: select a suggestion using the down arrow, then press tab to highlight the first one-off button. At this point the suggestion becomes gray and the first one-off button gets a blue highlight. Now press shift+tab. The current almost-specified behavior is shift+tab will close the panel and return the focus to the location bar. I think a more expected behavior (from a user point of view) would be to make the selected suggestion blue again, and either make the selected one-off gray, or unselect it. I would be happy to discuss this with Philipp if we are likely to finish this soon.