Added removeAll to simplify reference cleanup.

Currently the simplest way to stop FlxMouseEventManager from handling events (without having to individually remove each sprite that was added) was the effective, but ugly:

        new FlxMouseEventManager();

I just got tired of seeing that call in my FlxState.destroy() functions which doesn't express at all the intent (cleanup references to objects I'm destroying).  So I propose the the following instead:

        FlxMouseEventManager.removeAll()