Add a preference for setting the search engine

This combo box allows the user to select from:

* DuckDuckGo, Google, and Bing, always.
* Any smart bookmarks the user may have added.
* The current value of the gsetting, if the user has manually edited it.

An improvement would be to provide a better UI for the creation of smart
bookmarks in the preferences dialog.

https://bugzilla.gnome.org/show_bug.cgi?id=709459