Use QLineEdit's clear button feature in in Qt Designer.

Remove old filter widget.

Change-Id: Id65eee844d85df2d3c5a87f5acc26ba154d202c9
Reviewed-by: David Faure (KDE) <faure@kde.org>
Reviewed-by: Kevin Ottens <kevin.ottens@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>