Add a notication service to inhibit certain notifications based on metadata

Summary:
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.

Test Plan:
Wrote relevant patch for plasma-nm
Didn't get notification toggling a network whilst the popup was open
Did after I closed the popup

Reviewers: Plasma, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3589