Add a simple implementation of EventTarget interface on JS side
Add a simple implementation of EventTarget interface on JS side

To help extension developers reuse this implementation (or others),
a method named 'aggregate' is added to splice the chain of prototypes.

For example:
  exports.aggregate(Extension.EventTarget);
or for short because EventTarget impl is built-in:
  exports.aggregate('EventTarget');

TODOs:
* Native side support for EventTarget
* Sample code

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