A new parameter is defined :
```
chill_custom_fields:
show_empty_values_in_views = true|false
```
A new method is added to CustomFieldInterface: `isEmptyValue`. To ease
the dev of new classes, an AbstractCustomField class is created, which
implements the most commons function (currently, only isEmptyValue).
A new Twig Filter is added: `chill_custom_field_is_empty`
The twig filter `chill_custom_fields_group_widget` has a new possibility
in array option : `show_empty`. Default to
chill_custom_fields.show_empty_values_in_view. May be forced by
true/false.
The `custom field long choice` aim to provide a way to deal with choices
with a big possibilities.
The `custom field long choice` allow :
- to persist different options in the database ;
- each option has a key, a text (translatable string), and eventually a
parent, and an internal_key
- every key can be activate or not. If the parent is inactivated, all
childs are inactivated
- the internal key have two purposes :
- link to an external csv file, with their own key ;
- add a special class to results, to allow custom layout.
Currently, the field exists, but some elements are missing :
- a script for CSV import
- possibility to select multiple items
- edition of options
- handle of option without parents
- tests are missing
