Add ability to change notification sounds

[deadlydud]

Under settings perhaps under Sound Alerts or Notifications:
Thus to choose own sounds for notifications, pings, incoming rings, etc..
If overrided sound not found: fall back to default.

[zigpress]

No feedback on this for over a year?

This is vital - the tiny "chirp" on the macOS app when a new message comes in is too easily drowned out by other sounds in the environment.