Use QLineEdit's clear button feature in Active Qt.

Remove old filter widget.

Change-Id: I1ee7d8695d98044cc22e83fad81dd1f2b486b419
Reviewed-by: Kevin Ottens <kevin.ottens@kdab.com>
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>