Add a menu item to the dialogs sample for simulating having unsaved
documents in the application. Then demonstrate how to do something
when the user attempts to close the application, e.g. show a dialog,
and possibly cancel closing.
This code was originally written for debugging some issue related to
closing an application, but besides that it serves as an example of a
very common pattern a lot of real world applications need in some form.
Mention the example in samples/dialogs in wxCloseEvent documentation
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