[Widget Explorer] Show placeholder message when there are no widgets or search results

Summary:
BUG: 416313
FIXED-IN: 5.19.0

Test Plan:

Reviewers: VDG, Plasma, apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26715