Port Clipboard and Notifications applets to Kirigami.PlaceholderMessage

Summary:
The Clipboard and Notifications applets live in plasma-workspace; port them to use
Kirigami.PlaceholderMessage

Test Plan:
Clipboard: {F8253980}
Notifications: {F8253981}

Reviewers: #vdg, #plasma, broulik, niccolove, cblack

Reviewed By: #vdg, #plasma, niccolove, cblack

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T13021

Differential Revision: https://phabricator.kde.org/D29109