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/Websocket/MessageUserJoined.ts

@@ -0,0 +1,5 @@
+
+export class MessageUserJoined {
+    constructor(public userId: string, public name: string, public character: string) {
+    }
+}

back/src/Model/Websocket/MessageUserMoved.ts

@@ -0,0 +1,6 @@
+import {PointInterface} from "./PointInterface";
+
+export class MessageUserMoved {
+    constructor(public userId: string, public position: PointInterface) {
+    }
+}

back/src/Model/Websocket/PointInterface.ts

@@ -1,5 +1,5 @@
 export interface PointInterface {
-    x: number;
-    y: number;
-    direction: string;
+    readonly x: number;
+    readonly y: number;
+    readonly direction: string;
 }