Sieve: Add setting migration for switch to domain models

Author: janezd

Orange/widgets/visualize/owsieve.py

@@ -68,6 +68,7 @@ class OWSieveDiagram(OWWidget):
 
     want_control_area = False
 
+    settings_version = 1
     settingsHandler = DomainContextHandler()
     attr_x = ContextSetting(None, exclude_metas=False)
     attr_y = ContextSetting(None, exclude_metas=False)
@@ -119,6 +120,13 @@ def showEvent(self, event):
         super().showEvent(event)
         self.update_graph()
 
+    @classmethod
+    def migrate_context(cls, settings, version):
+        if not version:
+            settings.rename_setting("attrX", "attr_x")
+            settings.rename_setting("attrY", "attr_y")
+            settings.migrate_str_to_variable()
+
     def set_data(self, data):
         """
         Discretize continuous attributes, and put all attributes and discrete