Add an "Event" plugin to DarkConsole for event inspection

Summary: Shows events which a page dispatched, plus all the registered
listeners.

Test Plan:
Pretty basic for now, but works OK:

https://secure.phabricator.com/file/view/PHID-FILE-49fcd23081ce55cf9369/

(I also made it dispatch some dummy events to verify they show up.)

Reviewers: aran

Reviewed By: aran

CC: aran

Differential Revision: 973