Add caption to screenshot in AppStream data

Summary: …and bring Appstreamercli warnings on the CI down to zero.

Test Plan: appstreamcli validate --pedantic app/org.kde.gwenview.appdata.xml

Reviewers: Gwenview, ngraham

Reviewed By: Gwenview, ngraham

Differential Revision: https://phabricator.kde.org/D14033