QNSImageView - remove observer before dealloc

It can happen that NSMenuDidEndTrackingNotification comes after our imageCell
QNSImageView was deallocated (for example, mouse button is not released yet
when dealloc called). Remove soon-to-be-deallocated observer.

Change-Id: Ib155cc5f0b884c6b1fed0f986d12599096b582c6
Task-number: QTBUG-46425
Reviewed-by: Thierry Bastian <thierryb@filewave.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@theqtcompany.com>

src/plugins/platforms/cocoa/qcocoasystemtrayicon.mm

@@ -446,6 +446,7 @@ QT_END_NAMESPACE
 
 -(void)dealloc {
     [[NSStatusBar systemStatusBar] removeStatusItem:item];
+    [[NSNotificationCenter defaultCenter] removeObserver:imageCell];
     [imageCell release];
     [item release];
     [super dealloc];