Create a notifications service

Within the launcher create a service which other code can use to generate toast notifications. This requires #93 (closed) to be completed first.

Having a central place through which all notifications go will allow us to swap in alternative implementations in future, either for new versions of Windows or for different platforms (e.g. Linux)

Windows 10 Notifications: https://docs.microsoft.com/en-us/uwp/api/Windows.UI.Notifications.ToastNotificationManager

NuGet packages;

WindowsAPICodePack-Core
WindowsAPICodePack-Shell

Note: Requires a shortcut to exist on the menu. Perhaps add a warning in the installer that choosing not to have a start menu shortcut will disable notifications