Multi-monitor: don't limit core applets (sound, power, wifi) to single instance/panel

[Lurux]

When creating panels for a multi-monitor setup, users may be frustrated that some applets seem to be limited to a single instance per session - notably the volume, power and wifi applets.

On the surface, there doesn't seem to be any technical reason for this limitation, so allowing users to control their volume from more than a single panel would greatly improve the experience, as they'd otherwise have to use their "main panel" to control these basic options.

See also
https://github.com/orgs/linuxmint/discussions/1427

[Lurux]

Willing to help on that if the team is interested 😄

[Lurux]

Closing in favor of https://github.com/orgs/linuxmint/discussions/1429