Add API for opening the KCM to daemon

We have a few places that open the KCM, with different arguments.

Centralize the implementation in one place.

This makes it easier to switch to invoking systemsettings5 in the future (once plasma/systemsettings!11 is in).

It also makes sure the relevant device is selected when clicking on a pairing notification.

The function is exposed to DBus for the Plasmoid and potential third-party users.

CCBUG: 425660