Bigger slider and its handle

With embedded devices it is hard to hit the slider handle

Change-Id: If70a29a8794e34634d75b06744a780c674b102d1
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@theqtcompany.com>
Reviewed-by: Titta Heikkala <titta.heikkala@theqtcompany.com>
Reviewed-by: Pasi Keränen <pasi.keranen@theqtcompany.com>