Support for relations

- Adds an example on how to use a select field with OneOf/Enum values. 81656d8
- Adds a Deform widget for n:1 relations, e.g. selecting a district from a list of districts that are stored in a separate table. 975b5ae cd3abed
- Adds an example for 1:n relations, e.g. creating contact persons (with sub fields). d4953e2 35c65f3
- Adds a Deform widget for n:m relations, e.g. selecting multiple types from a list of types that are stored in a separate table. 4aaeea0 d05c1d0

Closes #7