feat(settings): make the tray icon optional

[atleugim]

Summary

• rename keep playing when closed to keep running in the background
• take the tray icon out when the app does not keep running in the background
• add a show in the system tray switch under it, on every platform
• remember the switch while background is off and restore it when background is on again
• skip fetching the cover for the tray menu while the icon is out

Draft: only tried on macOS so far. Still needs a run on Windows and Linux.

Solves #722