Add QQuickWindow::closing signal, and ability to ignore the event

An application can implement onClosing() and set
closeEvent.accepted = false to delay the closing (for example to
prompt the user to save changes).  Depends on change
I9abed47fca02a002b78727f98d678a824854adfc in qtbase.

Task-number: QTBUG-31019
Change-Id: Icfd4a03ecef3621bdbbee2e2c3157b897a9b6524
Reviewed-by: Alan Alpert (Personal) <416365416c@gmail.com>