Enums: store in JSON and database, implement matching Descriptor.

[eike-welk]

Problem

There are currently several collections of magic strings, that are used like Enums.

For example listing.type, listing.state, listing.site, listing.condition, also listing.curreny can be seen as an enum.

Arbitrary values can be put into their fields, use of enums would catch errors and API-changes.

Therefore

Use real enums instead of these magic strings.

• However these enums can not be simply stored in Databases or JSON files. Code that converts thes enums must be written.
• The current data descriptors also can not express enums. They must be extended to express enums.