Mark keyword-search-url default for translation

We allow region parameters in the default "Search the web" smart
bookmark, which probably nobody ever uses, so why not in the default
search engine that people actually DO use? Also needed because the DDG
URL in the preferences dialog will be translatable, and this must match
it to avoid being detected as a separate, custom search engine.

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