[Notifications] Also read .flatpak-info for identifying an application
Ensures we can properly map Flatpak'd apps to the correct service.
Spotify and VLC Flatpaks are now properly identified.
In the future will switch the heuristic around to try flatpak/snap before trusting what the client sent as most clients don't know their desktop file got mangled with in the process, so whatever the client sends for sandboxed apps is likely incorrect. And also, not trusting the client, if possible, is better anyway.
It is using QSettings instead of KConfig(..., SimpleConfig) beause for /proc/pid/root/.flatpak-info QFileInfo::canonicalFileName() returns /dev/null ?! utterly breaking KConfig's ini backend.