Fix styleData.activeFocus for TabViewStyle::tab

As the documentation says, the property is supposed to be true when
the specific tab button/item has active focus. It was an alias to tab
bar's activeFocus, so it was true for all tab buttons/items when any of
them had active focus.

Change-Id: I5fd56ec56bb496ce75735a63bbb581c4bcfc6186
Reviewed-by: Caroline Chao <caroline.chao@digia.com>