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 (merged) 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.