Add caption to screenshot in AppStream data

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

Test Plan: `appstreamcli validate --pedantic desktop/org.kde.spectacle.appdata.xml`

Reviewers: #spectacle, ngraham

Reviewed By: #spectacle, ngraham

Differential Revision: https://phabricator.kde.org/D14034