Add API Interface giving access to LB's Notification System

Currently on simple, non-UI involved plugins when e.g. a process/action is completed, we can notify the end user via a MessageBox or even System.Media.SystemSounds.Asterisk.Play();. These can be boring or in the case of a MessageBox, can get ‘lost’ behind LaunchBox’s UI.

Having access to LaunchBox’s Notification System for ‘simple messages’ will make for a clean, non-intrusive yet visible means of notifying/informing the end user.