Adding an API for inter-iframe communication

Adds a first version of an API to communicate between an iFrame opened by WorkAdventure and WorkAdventure itself.
The first API method is a method allowing to add messages in the chat, from the iFrame.

Comes with a test file.

front/src/Api/IframeListener.ts

@@ -0,0 +1,34 @@
+import {Subject} from "rxjs";
+
+interface ChatEvent {
+    message: string,
+    author: string
+}
+
+/**
+ * Listens to messages from iframes and turn those messages into easy to use observables.
+ */
+class IframeListener {
+    private readonly _chatStream: Subject<ChatEvent> = new Subject();
+    public readonly chatStream = this._chatStream.asObservable();
+
+    init() {
+        window.addEventListener("message", (event) => {
+            // Do we trust the sender of this message?
+            //if (event.origin !== "http://example.com:8080")
+            //    return;
+
+            // event.source is window.opener
+            // event.data is the data sent by the iframe
+
+            // FIXME: this is WAAAAAAAY too sloppy as "any" let's us put anything in the message.
+
+            if (event.data.type === 'chat') {
+                this._chatStream.next(event.data.data);
+            }
+
+        }, false);
+    }
+}
+
+export const iframeListener = new IframeListener();