[KCMs/Notifications] Port placeholder message to Kirigami.PlaceholderMessage

Test Plan: {F8273471}

Reviewers: #vdg, #plasma, broulik, filipf

Reviewed By: #vdg, filipf

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T13021

Differential Revision: https://phabricator.kde.org/D29296