Implement FieldWithoutContent and FieldWithContent for each of the supported types

[linxcat]

The String, Image, Video, Audio, RadioButton, CheckBox, and ListItem types must each get their own implementation of the two field abstractions

[williamvanderkamp]

I think for the ones with several options, we actually need all the options together to compose a single field.

i.e. RadioGroup, Checkboxes, List, instead of RadioButton, Checkbox, ListItem. (Though we do need a 'single option' case 'Checkbox' that is probably different from 'Checkboxes'.)

I will probably create a ListField interface that all of these 'several options' fields will implement.

FieldWithoutContent also needs a Serializable 'params' field so that the options and/or units can be passed with the spec. Each implementation of FieldWIthoutContent would know how to de/serialize its own params.