Draft: [kcms/tablet] Add combined tablet model

KWin/libinput splits tablet into separate tool and pad devices for technical reasons, and we expose that verbatim in the UI.

That leads to poor UX since for the user it is one physical device

This adds a model that combines devices that belong together into one logical device

This is done using the libinput device group property, which was created specifically for this purpose

Needs kwin!6060 (merged)