Refactored and optimized messages

Refactored and optimized messages

Now, when a user moves, only his/her position is sent back to the other users. The position of all users is not sent each time.

The messages sent to the browser are now:

- the list of all users as a return to the join_room event (you can send responses to events in socket.io)
- a "join_room" event sent when a new user joins the room
- a "user_moved" event when a user moved
- a "user_left" event when a user left the room

The GameScene tracks all these events and reacts accordingly.

Also, I made a number of refactoring in the classes and removed the GameSceneInterface that was useless (it was implemented by the LogincScene for no reason at all)