Add convenience wrapper to make customer field widget, etc.

customer widget is either autocomplete or dropdown, per config

also added a way to pass arbitrary kwargs to the chameleon template
rendering for a field

also moved the logic for rendering a <b-field> out of the template and
into the Form class

also start to prefer `input_handler` over `input_callback` when
specifying client-side JS hook

tailbone/views/customers.py

@@ -490,6 +490,11 @@ class CustomerView(MasterView):
     def configure_get_simple_settings(self):
         return [
 
+            # General
+            {'section': 'rattail',
+             'option': 'customers.choice_uses_dropdown',
+             'type': bool},
+
             # POS
             {'section': 'rattail',
              'option': 'customers.active_in_pos',