The code in the example showing how to create a new event class
confusingly used several names for the event type.
Fix this and also prefix the name of the new event type with "EVT_" to
match wxWidgets convention for naming event types.
Also explicitly mark the parts that are not needed if only Bind() is
used for event handling.
Closes https://github.com/wxWidgets/wxWidgets/pull/1586
8cf8c918b9