[enet] activate ENet wrapper
remove lib/ directory (unused)
[sdl2] removed SDL2CommandQueue.
       Its functionality is now assumed by the SDL2 class.

       Registering and unregistering windows is now unneeded.

       SDL Event can be polled, and before the poll returning the event:
       - kayboard state will update
       - the same event will be dispatched to window if it's a window event

       So you have some choice if you don't want to subclass Window.

added SDL2Window.getSize() method