Event/Growl notifications on client desktop machines

Allow Jamulus optionally to send visual event notifications using the host OS's notification function.

Examples including but not necessarily limited to:

- Player entering/leaving server
- Chat message notifications

Which events (or event types) to show should be configurable in Settings.

No files, tests, or entry points are named. Start by locating Jamulus's desktop event handling and Settings implementation, then determine how host-OS notifications can cover player presence and chat events. Done means optional notifications work for configurable event types across the supported desktop hosts.

Written by the indexing model from the issue text.