Merge the notification part of https://github.com/vector-im/riot-web/pull/2960
* Show and focus the window when the notification is clicked,
rather than just restoring it.
* Implement requestNotificationPermission and return a resolved
promise (although in practice it should never be called)
* Dismiss the notification when it's clicked
* Accept the room object: we need it to view_room
* Desktop app: Un-minimise when notification clicked
* Desktop App: Set App User Model ID so notifications work on
Windows 8!