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/templates/customers/configure.mako

@@ -3,6 +3,20 @@
 
 <%def name="form_content()">
 
+  <h3 class="block is-size-3">General</h3>
+  <div class="block" style="padding-left: 2rem;">
+
+    <b-field message="If not set, customer chooser is an autocomplete field.">
+      <b-checkbox name="rattail.customers.choice_uses_dropdown"
+                  v-model="simpleSettings['rattail.customers.choice_uses_dropdown']"
+                  native-value="true"
+                  @input="settingsNeedSaved = true">
+        Show customer chooser as dropdown (select) element
+      </b-checkbox>
+    </b-field>
+
+  </div>
+
   <h3 class="block is-size-3">POS</h3>
   <div class="block" style="padding-left: 2rem;">