Added simple state transitioning flow

* Changed all event/draw calls to use RenderWindow, as intended (my
bad).
* Press space to start a transition to a new state.
* Still needs some discussion and work, so I added some new TODOs.
Discuss best practices when working with TODOs.
* Fade out uses sprite and texture stored in System. Works, but feels
ugly. Discuss.
* State transition does not actually transition to a new state. As
intended! :)