erp5_autocompletion_list: Move away generic JS code from RelationField as it should be reusable anywhere.
erp5_autocompletion_ui: Allow customisation of object property being showed with autocompletion.

Possible through defining a TypeBasedMethod (PORTAL_TYPE_getCompletionDict)
returning a dict serialized in JSON by the calling method, or otherwise
overriding fallback script (Base_getCompletionDict).
erp5_autocompletion_ui: Rename ERP5Site_getCompletionList for naming consistency sake.
erp5_autocompletion_ui: Consider that description may not be needed, thus only one line, the label, is displayed.
erp5_autocompletion_ui: Autocompletion property value set to the field should be the one queried from the Catalog.
erp5_autocompletion_ui: Add JS ERP5Autocomplete() parameter to transform text before sending AJAX request.

For example, if one wants to uppercase/lowercase the value entered by
end-user.