Backport: Add Windows-specific screenshots as screenshots with custom attribute
This is a backport of !1163 (merged). I have taken care to keep the existing translations (i.e. the additional "ia" translation).
Instead of using translated custom values (which are not accepted by AppStream validation) this adds the Windows-specific screenshots as normal screenshots with custom attribute x-kde-os="windows". AppStream validation accepts custom attributes (in fact, it ignores any attributes other than "type").
Upstream appstream tooling will treat those screenshots as any other extra screenshots (screenshots without type="default").
(cherry picked from commit 6f4d2c02)