[libnotificationmanager] Port to lighter KProcessList

Summary:
KSysGuard fetches everything you could possibly need from a process.

If we just need a name of a process, we can use the new KProcessList API
in KCoreAddons.

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22327