NEW: Expose TreeDropdownField root node ID in schema

Forms part of a fix for silverstripe-admin#954

src/Forms/TreeDropdownField.php

@@ -901,6 +901,7 @@ class TreeDropdownField extends FormField
         $data['data'] = array_merge($data['data'], [
             'urlTree' => $this->Link('tree'),
             'showSearch' => $this->getShowSearch(),
+            'treeBaseId' => $this->getTreeBaseID(),
             'emptyString' => $this->getEmptyString(),
             'hasEmptyDefault' => $this->getHasEmptyDefault(),
             'multiple' => false,