Allow suppression of events (annotation groups)

It should be possible to remove an event type, when there are no annotations associated with it, or when the user confirms that all occurrences associated with this event can also be deleted.

A sensible way of doing this is to add a Delete button at the ChangeEvent dialog. Notice that this dialog is also used when choosing a new event is being created. In this case, the Delete button should not be added.

This will prevent events that were wrongly created to appear in the config.yml file of the project.