Remake scene state machine using a stack

[FranzPoize]

There is several problem with the current implementation

• It is not flexible there is only one scene and no way to partially cleanup a scene
• Scene are instantiated at the game startup which leads to a complicated lifecycle

What the new state machine should look like:

• [ ] a scene stack that can be poped and pushed onto
• [ ] a transition system that should be able to transfer information between scene if necessary
• [ ] scenes should be instantiated when they're needed and destroyed when removed from the stack

The last point leads to an issue of instantiating a lot of system at scene instantiation which should be taken care of.