Add Yandex search engine entry

[Fot0n]

Added an entry for Yandex to the list of default search engines.

Yandex ranked as the 4th largest search engine worldwide, based on information from Comscore.com
http://en.wikipedia.org/wiki/Yandex

[Emdek]

I think that we would need a mechanism to set defaults depending on locale, as there are a lot of search engines by default already.
With such mechanism we could do stuff like setting localized version of Wikipedia and enable one or more search engine popular for specified locale and hide less common ones like, Yahoo for example.

[BorutAtGit]

Whatever the final implementation around default search engines will be, please consider to also include Qwant, at least for European locales:

https://www.qwant.com/

(I am using it for a long time as a replacement for Google and DuckDuckGo.)