[Notifications] Introduce "settings" action

We've been having the configure button on notifications for a long time but so far it would
only ever invoke KNotification configuration UI with no way for an application to provide
its own, more versatile or extensive, UI.

When an action named "settings" is present in the notification the "configure" button will
instead invoke this action.

Differential Revision: https://phabricator.kde.org/D5424