SplitView: calculate implicit size
SplitView: calculate implicit size

Set the implicit size of the SplitView based on the sizes of the
split items.

Change-Id: I775b938215216831d10cb50868e4c6733e157eeb
Reviewed-by: Jan Arve Sæther <781b533117cd4e86bcc65e6d7bba551952271382@digia.com>

mizukusak/qtquickcontrols,mizukusak/qtquickcontrols,kobolabs/qtquickcontrols,drgvond/qtquickcontrols,kobolabs/qtquickcontrols,drgvond/qtquickcontrols,drgvond/qtquickcontrols,mizukusak/qtquickcontrols,kobolabs/qtquickcontrols