Signals: Fix some regressions from #36426

- Fix `callable_mp` bindings to methods which used to have default
  arguments passed to `bind_method`. We now have to re-specify them
  manually when connecting.
- Re-add `GroupsEditor::update_tree` binding.
- Misc code quality changes along the way.

editor/groups_editor.cpp

@@ -659,6 +659,7 @@ void GroupsEditor::_show_group_dialog() {
 }
 
 void GroupsEditor::_bind_methods() {
+	ClassDB::bind_method("update_tree", &GroupsEditor::update_tree);
 }
 
 GroupsEditor::GroupsEditor() {