cocoa: avoid sending windowNotification to a non-QNSView

Then it's possible to embed native cocoa widgets without crashing.

Task-number: QTBUG-35629
Change-Id: Ic212c36178282b4d090c6ce0470012adc8e0c2bb
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>