[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:
Reviewers: VDG, Plasma, broulik
Reviewed By: Plasma, broulik
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D29799