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:

Reviewers: VDG, Plasma, dvratil

Reviewed By: dvratil

Subscribers: plasma-devel

Tags: Plasma

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