Cocoa: do not send multiple window activation events

Cocoa: do not send multiple window activation events

When switching the active window within the same application, Cocoa
sends us an NSWindowDidResignKeyNotification for the old activated
window, then an NSWindowDidBecomeKeyNotificationfor the newly activated
window. Our handling of this would first set Qt's active window to zero,
then immediately reset it afterwards. Avoid this by checking the key
window when handling the deactivation event, and don't set the active
window to zero if a new window has become active.

Task-number: QTBUG-24322
Change-Id: I8719fc501049eeaaebb75e9ea03261b2209458b6
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>