Port placeholder message to Kirigami.PlaceholderMessage

Summary:
This makes the appearance more standardized by using the new Kirigmai library component.

Depends on D29057

Test Plan: {F8251670}

Reviewers: #vdg, #plasma, dvratil

Reviewed By: dvratil

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29060