Add Windows-specific screenshots as screenshots with custom attribute

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)