Fix keeping selection value in a selection field

Previously, the value of the selection was lost in the request and, if the
form field was marked as required, then a ticket could never be submitted.

include/class.dynamic_forms.php

@@ -708,6 +708,8 @@ class DynamicListItem extends VerySimpleModel {
 }
 
 class SelectionField extends FormField {
+    static $widget = 'SelectionWidget';
+
     function getListId() {
         list(,$list_id) = explode('-', $this->get('type'));
         return $list_id;
@@ -719,10 +721,6 @@ class SelectionField extends FormField {
         return $this->_list;
     }
 
-    function getWidget() {
-        return new SelectionWidget($this);
-    }
-
     function parse($value) {
         return $this->to_php($value);
     }