In most scenarios, zoom change processing happens asynchronously. In
consequence, widgets may evolve and even already be disposed until a
notification for a zoom change event on them actually is processed. This
can lead to disposed widget exceptions in case such a notification is
sent on an already disposed widget.

This change adapts all places that send zoom change notification on DPI
change to check for disposal of the widget to send the notification to
first.

Fixes https://github.com/eclipse-platform/eclipse.platform.swt/issues/3043