Cleaned up Client interface.

-- Unexposed worldChanged (this should not be allowed for plugins).
-- Removed activeCamera (use entity.getType(Camera.class) or an extension
   of the camera interface you may have).
-- Remove redundant active naming. This will make more sense when the
   Engine interface no longer has the getPlayer/getWorld methods (they are
   going into Server).
-- Added a convienent getWorld which is always the world the client player
   is in as well as the currently rendererd world.
Signed-off-by: Chris Sanders <zidane@outlook.com>