function showSaveDialog(title, initialPath, proposedNewFilename, callback) {
        appshell.fs.showSaveDialog(title, initialPath, proposedNewFilename, _wrap(callback));
    }
Display a save-file dialog and call back asynchronously with either a
FileSystemError string or the path to which the user has chosen to save
the file. If the dialog is cancelled, the path string will be empty.

@param {string} title
@param {string} initialPath
@param {string} proposedNewFilename
@param {function(?string, string=)} callback