[mod] move language recognition to get_search_query_from_webapp

To set the language from language recognition and hold the value selected by the
client, the previous implementation creates a copy of the SearchQuery object and
manipulates the SearchQuery object by calling function replace_auto_language().

This patch tries to implement a similar functionality in a more central place,
in function get_search_query_from_webapp() when the SearchQuery object is build
up.

Additional this patch uses the language preferred by the client, if language
recognition does not have a match / the existing implementation does not care
about client preferences and uses 'all' in case of no match.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>