SplitView: rename 'expanding' to 'fillWidth'
SplitView: rename 'expanding' to 'fillWidth'

Rename variables to the new Layout terminology, to ease
future maintenance/understanding of the code.

Change-Id: I619f62a0280238fafefe261fae3f00eb575bb699
Reviewed-by: Jan Arve Sæther <781b533117cd4e86bcc65e6d7bba551952271382@digia.com>

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