Make screenshots visible when there's only one screenshot too

Summary:
Applications with only one screenshot also deserve to have it visible.

BUG: 371724
FIXED-IN: 5.8.4

Test Plan:
Open any application that only has one screenshot (e.g. poedit).
No screenshot is shown until this patch is applied.

Reviewers: davidedmundson, apol

Reviewed By: davidedmundson, apol

Differential Revision: https://phabricator.kde.org/D3223