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)

back/src/Model/World.ts

@@ -48,6 +48,10 @@ export class World {
         return this.groups;
     }
 
+    public getUsers(): Map<string, UserInterface> {
+        return this.users;
+    }
+
     public join(socket : Identificable, userPosition: PointInterface): void {
         this.users.set(socket.id, {
             id: socket.id,
@@ -74,8 +78,7 @@ export class World {
             return;
         }
 
-        user.position.x = userPosition.x;
-        user.position.y = userPosition.y;
+        user.position = userPosition;
 
         if (typeof user.group === 'undefined') {
             // If the user is not part of a group: