Add Notification support to the Web Platform

Except display notification which was already accidentally included in a
previous PR

src/vector/platform/WebPlatform.js

@@ -63,6 +63,40 @@ export default class WebPlatform extends BasePlatform {
         this._updateFavicon();
     }
 
+    /**
+     * Returns true if the platform supports displaying
+     * notifications, otherwise false.
+     */
+    supportsNotifications() : boolean {
+        return true;
+    }
+
+    /**
+     * Returns true if the application currently has permission
+     * to display notifications. Otherwise false.
+     */
+    maySendNotifications() : boolean {
+        return global.Notification.permission == 'granted';
+    }
+
+    /**
+     * Requests permission to send notifications. Returns
+     * a promise that is resolved when the user has responded
+     * to the request. The promise has a single string argument
+     * that is 'granted' if the user allowed the request or
+     * 'denied' otherwise.
+     */
+    requestNotificationPermission() : Promise {
+        // annoyingly, the latest spec says this returns a
+        // promise, but this is only supported in Chrome 46
+        // and Firefox 47, so adapt the callback API.
+        const defer = q.defer();
+        global.Notification.requestPermission((result) => {
+            defer.resolve(result);
+        });
+        return defer.promise;
+    }
+
     displayNotification(title: string, msg: string, avatarUrl: string) {
         const notification = new global.Notification(
             title,