[Notifications] Report effective Inhibited state on DBus

The inhibition handling (e.g. combining time-based, screen mirrored, etc) is done inside the applet.
The Server knows nothing about it and only reports Inhibited as true when an external application
requested one, not when the user enabled it in the applet.
This patch exposes the NotificationManager.Server as singleton QML type and adds a way for the
applet to tell it the effective inhibition state.
Exposing the server to QML could also be used in the future to provide better error reporting
to the user when the service isn't running and/or owned by someone else (e.g. Dunst)

Differential Revision: https://phabricator.kde.org/D24486