function getCurrentlyViewedPath(paneId) {
        var file = getCurrentlyViewedFile(paneId);
        return file ? file.fullPath : null;
    }
Retrieves the currently viewed path of the pane specified by paneId
@param {?string} paneId - the id of the pane in which to retrieve the currently viewed path
@return {?string} the path of the currently viewed file or null if there isn't one