Fixed NPE in NotificationListView SelectionChangedEventHandler.

When the list store clears, or if a user happens to ctrl/cmd+click a selected notification, this SelectionChangedEvent handler would throw an NPE.
DEViewImpl will now fetch last 10 notifications initially.

Refactored initialization of the ViewNotificationMenu into a new buildNotificationMenu method.
This method makes an initial call to ViewNotificationMenu#fetchUnseenNotifications so the menu will be populated from the start.