Limit the length of shown search result line.

This e.g. avoids "lines" of several MB if file search hits a binary
file.

Task-number: QTCREATORBUG-6116
Change-Id: Ibf89a332bc82ddbdb2aeb83c4b3886e784dab4b8
Reviewed-by: Ralf Neubersch <neubersch@neubersch.de>
Reviewed-by: Robert Löhning <robert.loehning@nokia.com>