Improved gallery listing

Summary: The gallery listing is now a CardsLayout of cards with images and has a search bar.

Test Plan: {F8225697}

Reviewers: #vdg, #kirigami, mart

Reviewed By: #vdg, #kirigami, mart

Subscribers: plasma-devel

Tags: #plasma, #kirigami

Differential Revision: https://phabricator.kde.org/D28671