Make URLbar on the bottom optional through about:config

This MR proposes a way to fix #51 (closed), as outlined in #51 (comment 453942) To move it back up, just adding mcf.addressbarontop as a boolean value in about:config and set it to true.

Q: Why is the setting prepended with "not"?

A: I tried to set an opposite value as true via mobile-config-prefs.js and mobile-config-autoconfig.js, and it would not work. Hence this approach, to keep the 'stuff on the bottom' default.

PS: If this ends up working out, we could also use the same approach for issues like #42 (closed), #34 (closed), #56. Better yet, if we manage to find a way to set values without users entering `about:config (e.g., via something like Add Water, an extension or ...), we can provide more flexible and maybe make some forks or patches in packaging unnecessary.