Add Windows-specific screenshots as screenshots with custom attribute

This replaces the interim solution added with !1060 (merged).

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"), i.e. they may be show by Linux app stores. Adding more Linux screenshots with type="default" would move the extra screenshots further out of sight.