Viewport: introduce the fullscreenchange event

docs/api/viewport.md

 **Example**
 
 ```js
-viewport.addEventListener('resize', event => {
+viewport.addEventListener('resize', () => {
     console.log('The viewport has been resized.');
 });
-```
+```
+
+### fullscreenchange
+
+The viewport has been switched into or out of fullscreen mode.
+
+*Since:* 0.3.0
+
+**Example**
+
+```js
+viewport.addEventListener('fullscreenchange', () => {
+    if(viewport.fullscreen)
+        console.log('Switched into fullscreen mode');
+    else
+        console.log('Switched out of fullscreen mode');
+});
+```

src/core/viewport.ts

 type ViewportSizeGetter = () => SpeedySize;
 
 /** All possible event types emitted by a Viewport */
-type ViewportEventType = 'resize';
+type ViewportEventType = 'resize' | 'fullscreenchange';
 
 /** An event emitted by a Viewport */
 class ViewportEvent extends AREvent<ViewportEventType> { }
  */
 class ViewportFullscreenHelper
 {
+    /** The container to be put in fullscreen */
+    private readonly _container: HTMLElement;
+
+
+
     /**
      * Constructor
-     * @param _container the container which will be put in fullscreen
+     * @param _viewport Viewport
      */
-    constructor(private readonly _container: HTMLElement)
+    constructor(private readonly _viewport: Viewport)
     {
+        this._container = _viewport.container;
     }
 
     /**
                     if(container === (document as any).webkitFullscreenElement) {
                         Utils.log('Entering fullscreen mode...');
                         resolve();
+                        this._triggerEvent();
                     }
                     else
                         reject(new TypeError());
             }).then(() => {
                 Utils.log('Entering fullscreen mode...');
                 resolve();
+                this._triggerEvent();
             }, reject);
         });
     }
                     if(doc.webkitFullscreenElement === null) {
                         Utils.log('Exiting fullscreen mode...');
                         resolve();
+                        this._triggerEvent();
                     }
                     else
                         reject(new TypeError());
             document.exitFullscreen().then(() => {
                 Utils.log('Exiting fullscreen mode...');
                 resolve();
+                this._triggerEvent();
             }, reject);
         });
     }
         else
             return false;
     }
+
+    /**
+     * Trigger a fullscreenchange event
+     */
+    _triggerEvent(): void
+    {
+        const event = new ViewportEvent('fullscreenchange');
+        this._viewport.dispatchEvent(event);
+    }
 }
 
 
         this._resizer = new ViewportResizer(this);
         this._resizer.setStrategyByName(this._style);
 
-        this._fullscreen = new ViewportFullscreenHelper(this.container);
+        this._fullscreen = new ViewportFullscreenHelper(this);
     }
 
     /**