Help: Fix highlighting of search terms.

It was using the wrong search help engine to get the search terms.
Instead let the search widget itself figure out the search terms, and
pass it directly on to the help widget when activating links.

Change-Id: Ib72220eafca69e92894fe7584b1e4dfd9a42cc7e
Task-number: QTCREATORBUG-13239
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Reviewed-by: Christian Stenger <christian.stenger@digia.com>