Let slider have discrete steps when accessible

Change-Id: I3197104932dfd6e9dd9516343be7313ced4b6090
Task-number: QTBUG-41989
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>
"function accessiblePressAction()" => "Accessible.onPressAction:"

Change-Id: Ic9f8bdf531845a77e43cf526eb9256b85787da5b
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>