Do not include hidden items in layouts.

Note that hidden items also excludes their spacing. So there is no
double-spacing due to this.
However, items with size of 0 (duh) will have double-spacing.
(This is to avoid sudden "jumps" in the layout if the item changes
size from 1 to 0)

Change-Id: Ib170dc8c575cbd8bbe8bb28de7fa64ae6ab5062d
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>