macOS: Warn the user when KVO-observing QCocoaNSWindow

We recreate the window of our QNSView in various situation such as changing
the styleMask, which results in either a new QNSwindow or QNSpanel being
set as the window of the view. KVO-observing the window property of the
view will fail we then have a new window instance, and will crash when
trying to remove the observer from the new window.

Unfortunately catching the NSRangeException that happens when removing
an observer from a 'switched' window is not possible, so the user is
left with:

  Terminating app due to uncaught exception 'NSRangeException', reason:
    Cannot remove an observer <Foo 0xabc> for the key path "bar"
    from <QNSPanel 0xdef> because it is not registered as an observer.

Change-Id: Ib2adbb99f19303a054fb1eb65e959aecd32b1162
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

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