Port "sorry, nothing found" message to Kirigami.PlaceholderMessage

Test Plan: {F8313851, size=full}

Reviewers: apol, #discover_software_store, #vdg, filipf

Reviewed By: #vdg, filipf

Subscribers: filipf, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29689