[FEATURE] Concatenate list fields

__Tasks:__

- [x] test coverage
- [x] gather feedback for my changes
- [x] submit changes to the documentation

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | parts of #129 
| Doc           | none
Yes, but I still prefer the other way... IMHO it's easier to use if you choose the correct sort order and default. That means that if you want to change the glue you can write `new FieldDescriptor(array(), 'myname', ', ')` instead of putting the last parameter into an own array with a key.