This is useful for implementing "one shot" events.
The property can be set via the constructor or setter.
If 'handle_once' is set to True, then the EventHandler unregisters itself from the context after the first time it is handled.
All subclasses of EventHandler pass it kwargs so they can use the new property.
Tests included.

A common pattern for describing event handlers has been moved to the parent class.
Subclasses should implement the 'handler_description' and 'matcher_description' properties rather than implementing a describe method.

Plus other minor cleanup.