Add API for showing the platform's notification configuration

This is particularly relevant for sandboxed applications, as those cannot change how the system presents their notifications themselves.

For the sandboxed case this works for Flatpak on Plasma >= 6.7 and on Android. For the non-sandboxed case there's also a fallback to kcmshell, which should make this work in most cases. Applications can also query via API whether this is going to work, to only show the corresponding action in that case.

This is based on similar code in Itinerary.