ProxyEvent support to generic events

This patch aims to add the support to an additional class parameter to the ProxyEvent annotation.

There's a discussion at the forum about the topic: https://groups.google.com/forum/#!topic/gwt-platform/VKZupHPgeaM

This optional parameter is intended to be used for generic events when firing each specific type to only its handler type is necessary.

The pattern is better described here: https://groups.google.com/forum/#!searchin/google-web-toolkit-contributors/generic$20events/google-web-toolkit-contributors/Q5lO5-ORZw4/FS5u6o7mbKcJ