Call base class init() from QSGListViewPrivate

Call base class init() from QSGListViewPrivate

Regression from 9b5f9c8056186c9cf3c40dbac66e5f90369339dc when
QSGItemView was created.

Also fixed other reimplemented methods in QSGListViewPrivate to call
their base class functions (which are currently empty) and changed
some virtual methods in QSGItemViewPrivate to non-virtual where they
didn't need to be virtual.

Task-number: QTBUG-20679
Change-Id: I489593b3eab4f5bbb4126906f170342d1e8811cd
Reviewed-on: http://codereview.qt.nokia.com/2462
Reviewed-by: Bea Lam <bea.lam@nokia.com>
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>