Extract custom values with screenshot captions
AppStream does not support platform-specific screenshots. We add them as custom values together with captions/descriptions as for "normal" screenshots. To extract them we need to look for custom values with specific values for the key attribute.
See brief discussion of different options at https://mail.kde.org/pipermail/kde-i18n-doc/2023-July/001646.html
The big picture is that we want to fill all metadata for app submissions in the Microsoft Store with information taken from the apps' AppStream data.