[Notifications KCM] Show hint if notifications aren't available or provided by someone else

I've seen people complaining about ugly notification popups when they were in fact running Dunst instead of Plasma.
Since the KCM won't work with notifications other than Plasma's, this patch adds a hint informing the user
when there is no notification widget or notifications are provided by another service than Plasma.
Notification API provides a way to query server identity which is used to determine if it's Plasma or not.
The process name isn't used as you could have a notifications widget in e.g. Latte Dock.

Differential Revision: https://phabricator.kde.org/D25097