StackView: rename currentPage to currentItem
StackView: rename currentPage to currentItem

Change-Id: Id2d1d3339e36d8b6d8c4e65c775d2675fde81798
Reviewed-by: Jens Bache-Wiig <9cea9d70b08f8210134ec5959114f72bad51ed1d@digia.com>

kobolabs/qtquickcontrols,drgvond/qtquickcontrols,kobolabs/qtquickcontrols,jfaust/qtquickcontrols,jfaust/qtquickcontrols,drgvond/qtquickcontrols,richardmg/qtcomponents,mizukusak/qtquickcontrols,richardmg/qtcomponents,drgvond/qtquickcontrols,richardmg/qtcomponents,mizukusak/qtquickcontrols,jfaust/qtquickcontrols,kobolabs/qtquickcontrols,mizukusak/qtquickcontrols