ERP5Form: Returned field default value for ListBox was not consistent with non-ListBox field.

So, Relation Fields in ListBox were ignored during validation as the input
value is compared with the current (default), thus the field could not be
cleared as the value from the actual object was not retrieved at all.
Likewise a non-ListBox field, call custom edit() if available (eg encapsulated editor).

For example, MultiRelationStringField does not return a plain value, but a
MultiRelationEditor overriding Base_edit default behavior.
Add support for Listbox for getting Relation field field parameter dict.
erp5_autocompletion_ui: Serialization of array was broken with ERP5.
erp5_autocompletion_ui: Increase delay before searching from 300ms (default) to 600ms, this should be enough...
erp5_autocompletion_ui: Clean up and make parameters names consistent with ERP5.
RelationField: Implement autocompletion and use it by default once erp5_autocompletion_ui is installed.