Generate DBus interface

Summary:
We already use a generated interface in kstatusnotifieritem, but not in notifybypopup.

Compile-time checks++
code--

Test Plan: Spawned, updated, closed notifications, triggered actions

Reviewers: Frameworks, broulik

Reviewed By: broulik

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D29420