Added the call to set_database (neccessary for incremental search).

According to the documentation of xapian [1], I added a line of code that tells the QueryParser what database to use for the partial (incremental) search. Now it works!

[1] http://xapian.org/docs/apidoc/html/classXapian_1_1QueryParser.html#ae96a58a8de9d219ca3214a5a66e0407e