Debugger implementation idea

[gcao]

Define an event listener interface

When an event occurs, call registered event listener with event type and context Event type can be method_definition, advice_definition, method_invocation, advice_invocation, method_return, advice_return etc

Debugger implements event listener. It outputs message on handling event

A dot-based graph generator can be created and registered as event listener. It'll generate method+advices graph and execution graph.