[Approved] Plasma 6 Proposal: incubate SDDM

Being a non-KDE project necessarily imposes more distance and loses certain opportunities for tighter integration, like the kind that GNOME has with their GDM login manager. It would be of long-term benefit to tighten integration between SDDM and Plasma to improve the UX in a way that incubation within KDE could achieve.

At this point nearly all SDDM development comes from KDE devs anyway, so it's already sort of de facto incubated in all but name and release schedule. On that subject, being a part of presumably the Plasma release cycle would also ensure that SDDM gets regular releases, which it is desperately in need of.

Obviously we would need to talk with the prior maintainer Pier Luigi Fiorini about this.