Introduce PlaceholderMessage

This is a clone of the Kirigami component that we can use in Plasma applets until we arrive at a solution for styling QQC2 items using the Plasma style (https://phabricator.kde.org/T13256).

See dependent patches:

• plasma-workspace: plasma-workspace!73 (merged)
• bluedevil: bluedevil!1 (merged)
• print-manager: https://invent.kde.org/utilities/print-manager/-/merge_requests/1
• plasma-vault: plasma-vault!3 (merged)