[kcmkwin/kwinrules] Port placeholder message to Kirigami.PlaceholderMessage

Summary: Depends on D29152

Test Plan:
{F8256396}

{F8257685}

Reviewers: iasensio, #kwin, zzag

Reviewed By: iasensio, #kwin, zzag

Subscribers: kwin

Tags: #kwin

Maniphest Tasks: T13021

Differential Revision: https://phabricator.kde.org/D29141