Add documentation to all event classes

I've added comments to all the event classes. A lot of comments are similar to the ActionScript3 documentation.

Please review!

Here is a list of things that I found while writing the comments: 

* `BroadcastEvent`: Is this comment correct? The name `BroadcastEvent` is a bit misleading because the same term is used in Dart Streams. There are broadcast Streams and non-broadcast Streams (see `StreamController.broadcast`.
* `EventStream`: Did I write the correct comment for the `listen` and `capture` methods? Is the description of the capture, target, and bubbling phase correct?
* `MouseEvent`: As far as I could tell, the `clickCount` property is always 0 (see `Stage._onMouseEvent`). If this is correct, then we must change the comment of `clickCount` and indicate that it is always 0.