Improvement: Allow import of card image IDs from unknown sources

[chairobl]

The XML file used by the desktop tool processes image IDs hosted on any Google Drive, allowing for ordering self-made proxies.
However, the frontend does currently not support importing such altered files, replacing the IDs and issuing a warning

If there's interest I'd like to create a PR that adds an import option for ignoring unknown card images, which allows picking card backs and combining multiple XML files through the frontend while preserving unknown card image IDs:

This allows for sharing files with self-made IDs, e.g. for the purposes of a group order where one person combines multiple files from different people before ordering them through the desktop tool.