(core) Add RenameChoices user action

Summary:
["RenameChoices", table_id, col_id, renames]
Updates the data in a Choice/ChoiceList column to reflect the new choice names.
`renames` should be a dict of `{old_choice_name: new_choice_name}`.
This doesn't touch the choices configuration in widgetOptions, that must be done separately.
Frontend to be done in another diff.

Test Plan: Added two Python unit tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3050

sandbox/grist/test_engine.py

@@ -3,6 +3,7 @@ import functools
 import json
 import unittest
 from collections import namedtuple
+from pprint import pprint
 
 import six
 
@@ -203,7 +204,10 @@ class EngineTestCase(unittest.TestCase):
     """
     Prints out_actions in human-readable format, for help in writing / debugging tets.
     """
-    print("\n".join(self._formatActionGroup(out_actions.__dict__)))
+    pprint({
+      k: [get_comparable_repr(action) for action in getattr(out_actions, k)]
+      for k in self.action_group_action_fields
+    })
 
   def assertTableData(self, table_name, data=[], cols="all", rows="all", sort=None):
     """