[kcms/notifications] Port "no search result" message to PlaceholderMessage

Summary:
Now the appearance is consistent with other similar messages, as well as the
one to the right of it.

Test Plan: {F8326167}

Reviewers: #vdg, #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29799