[Feature #6562] add editable option for all edit types

I have added an option to define if a field is editable or not by the user.

I know there was the immutable edit type but, this property is more a field property than a proper edit type. Indeed, value relations, ranges or checkboxes could not be disabled before.

I have not removed the immutable type since the edit types are saved as numbers (and not text) in the project.
I would propose to change this. But for compatibility, we would have to create some translation methods (int->string, string->int) to traduce the enum type to the name of the edit type.
Discussion's open!