Add 'Event' (capital E) to browser variables - to support http://mzl.la/1mR23Bc

Replaces #1645

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Otherwise I get warnings for using undefined variables when using `Event.BUBBLING_PHASE`, `Event.AT_TARGET`, `Event.CAPTURING_PHASE` and `Event.NONE`

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@valueof said I needed to update the tests - but this is just a configuration change (adding just adding an allowed global) and I couldn't find any similar tests for `event` (lowercase), or, `ElementTimeControl`, or many of the other vars listed in that file.

Happy to add a test but couldn't find any suitable template to work from :-/.