code: function onActiveEditorChanged(event, current, previous) {
          if (current && !current._codeMirror._lineFolds) {
              enableFoldingInEditor(current);
          }
          if (previous) {
              saveLineFolds(previous);
          }
      }
docs: When a brand new editor is seen, initialise fold-gutter and restore line folds in it.
  Save line folds in departing editor in case it's getting closed.
  @param {object} event the event object
  @param {Editor} current the current editor
  @param {Editor} previous the previous editor