Fix DesignSpaceDocument lib serialization

[madig]

TODO:

• Actually fix serialization
• Should serialization also remove the "@" character from field names that serves only as a hint for quick-xml?

Closes #338.

[madig]

I now realize that the serializer is a special-case implementation for quick-xml. Should the XML serialization stuff be kept in a separate serializer maybe to not interfere with lib serialization?

[RickyDaMa]

Should the XML serialization stuff be kept in a separate serializer maybe to not interfere with lib serialization?

Yeah I have a feeling to have good XML & JSON support we'll probably need to separate the implementations, given how serde annotations etc. are used by quick-xml. Just a hunch, I haven't looked into this myself. It's that or we may have to bully serde into using different (de)serialization logic depending on the format (which AFAIK it's not really supposed to do)