Fix "current value" for <b-select> element in e.g. edit form views

apparently marking an `<option>` as "selected" does not cut it for Buefy, and
we must bind to a v-model somehow.  not real crazy about the current method,
but it does seem to work okay so far...

tailbone/templates/deform/select.pt

@@ -56,7 +56,9 @@
   </script>
   </div>
 
-  <div tal:condition="use_buefy" tal:omit-tag="">
+  <div tal:condition="use_buefy"
+       tal:define="vmodel vmodel|'field_model_' + name;"
+       tal:omit-tag="">
     <b-select tal:attributes="name name;
                               id oid;
                               placeholder '(please choose)';
@@ -64,6 +66,7 @@
                               multiple multiple;
                               size size;
                               style style;
+                              v-model vmodel;
                               @input input_handler;">
 
       <tal:loop tal:repeat="item values">

tailbone/templates/forms/deform_buefy.mako

@@ -95,6 +95,8 @@
                       field_model_${field.name}: null,
                   % elif isinstance(field.schema.typ, deform.FileData):
                       field_model_${field.name}: null,
+                  % else:
+                      field_model_${field.name}: ${json.dumps(field.cstruct)|n},
                   % endif
               % endif
           % endfor