Include completion search into SearchEngine

Include completion search into SearchEngine

By default it still uses PrefixSearch and supports PrefixSearchBackend
but it can be deprecated and phased out and SearchEngine extensions used
instead.

New APIs:
- SearchEngine
	public function defaultPrefixSearch( $search );
	public function completionSearch( $search );
	public function completionSearchWithVariants( $search );

Search engines should override:
protected function completionSearchBackend( $search );

Bug: T121430
Change-Id: Ie78649591dff94d21b72fad8e4e5eab010a461df