Sometimes we get notifications for events that are also displayed by
Plasma, giving us some rather annoying popups.
This patch provides a service on the notification dataengine so that
other plasmoids can block the notification applet from processing those
notifications.
Intended use case is for the network manager plasomid to not show a
notification whilst the dialog is open, as notifications come from kded.
However it's kept generic enough that other plasmoids can filter on
other hints, such as category or desktop file.