owpaintdata: Fix undo of add new class label

'undoable' param was passed from triggered signal (checked state).
This was wrong.

Orange/widgets/data/owpaintdata.py

@@ -1074,19 +1074,14 @@ def reset_to_input(self):
 
         self.commit.deferred()
 
-    def add_new_class_label(self, undoable=True):
-
+    def add_new_class_label(self):
         newlabel = next(label for label in namegen('C', 1)
                         if label not in self.class_model)
-
         command = SimpleUndoCommand(
             lambda: self.class_model.append(newlabel),
             lambda: self.class_model.__delitem__(-1)
         )
-        if undoable:
-            self.undo_stack.push(command)
-        else:
-            command.redo()
+        self.undo_stack.push(command)
 
     def remove_selected_class_label(self):
         index = self.selected_class_label()

Orange/widgets/data/tests/test_owpaintdata.py

@@ -13,6 +13,7 @@
 from orangecanvas.gui.test import mouseMove
 
 from Orange.data import Table, DiscreteVariable, ContinuousVariable, Domain
+from Orange.widgets.utils import itemmodels
 from Orange.widgets.data import owpaintdata
 from Orange.widgets.data.owpaintdata import OWPaintData
 from Orange.widgets.tests.base import WidgetTest, datasets
@@ -189,6 +190,32 @@ def assert_close(p1, p2):
         w.undo_stack.undo()
         assert_close(w.data[0], p0)
 
+    def test_add_remove_class(self):
+        def put_instance():
+            w.set_current_tool(owpaintdata.PutInstanceTool)
+            QTest.mouseClick(viewport, Qt.LeftButton)
+
+        def assert_class_column_equal(data):
+            assert_array_equal(np.array(w.data)[:, 2].ravel(), data)
+
+        w = self.widget
+        viewport = w.plotview.viewport()
+        put_instance()
+        itemmodels.select_row(w.classValuesView, 1)
+        put_instance()
+        w.add_new_class_label()
+        itemmodels.select_row(w.classValuesView, 2)
+        put_instance()
+        self.assertSequenceEqual(w.class_model, ["C1", "C2", "C3"])
+        assert_class_column_equal([0, 1, 2])
+        itemmodels.select_row(w.classValuesView, 0)
+        w.remove_selected_class_label()
+        self.assertSequenceEqual(w.class_model, ["C2", "C3"])
+        assert_class_column_equal([0, 1])
+        w.undo_stack.undo()
+        self.assertSequenceEqual(w.class_model, ["C1", "C2", "C3"])
+        assert_class_column_equal([0, 1, 2])
+
 
 class TestCommands(unittest.TestCase):
     def test_merge_cmd(self):  # pylint: disable=import-outside-toplevel