Refactor FieldSettingsForm, getConfguration and setConfiguration

Problem/Motivation

Discussed with @Berdir, there is problem in FieldDiffBuilder with configuration. Namely methods getConfiguration and setCOnfiguration gives an object instead of array (as indicated in documentation).

Proposed resolution

Make getter and setter of configuration return an array as written in class documentation instead of an object and amek refactoring of other classes in order to adopt that chnage.

Remaining tasks

User interface changes

API changes

Data model changes