Fix Slider regression

The change in implicit size means that by default the size is not
0. Since the slider was creating a hidden slider item, we have to
hide this explicitly.

Change-Id: I5342376ab209409a580b9380deff6f53b0d94f63
Reviewed-by: Daker Fernandes Pinheiro <daker.pinheiro@openbossa.org>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@nokia.com>