Slightly larger slider size

When using tick marks we clip the slider at the bottom
in both mac and gtk style. Even on windows the handle
is too small so I will increase the default size a bit.

Change-Id: Ief2cab5f8c9edbc8c1966530a0c3b52234c30796
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>