function onActiveEditorChanged(event, current, previous) {
        if (current && !current._codeMirror._lineFolds) {
            enableFoldingInEditor(current);
        }
        if (previous) {
            saveLineFolds(previous);
        }
    }
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