add required schema to pointingdocs, to allow editing

themis/fields/interfaces.py

@@ -188,6 +188,20 @@ class IPreviewDocWidget(IWidget):
 
 class IPointingDocs(IField):
     '''Field to display a list of documents pointing to the current one'''
+    required = schema.Bool(
+        title=IField['required'].title,
+        description=IField['required'].description,
+        default=False)
+
+    default = schema.TextLine(
+        description=IField['default'].description,
+        required=False)
+
+    missing_value = schema.TextLine(
+        title=IField['missing_value'].title,
+        description=IField['missing_value'].description,
+        required=False)
+
 
 class IPointingDocsWidget(IWidget):
     pass