Cocoa: Don't send duplicate close events.

Don't send QCloseEvents to QWidgetWindows during
cmd-q application shutdown, since widgets will
will already have received close events from
QApplication close event handling.

Task-number: QTBUG-39398
Change-Id: I7f6e892b0042361bed7a3bc5fac8518eabfc8e4e
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>