Add ability for plugins to notify users of events through Windows 'toasts' notifications

[bjsi]

Enable plugin writers to notify users of events.

• Notify the user if an external service / API isn't responding.
• Could be used to communicate more interesting information like Pomodoro time tracking info, information from Watcher.
• Users should be able to set notification preferences in plugin configs.

[alexis-]

See https://github.com/supermemo/SuperMemoAssistant.Services.ToastNotifications