function handleFileCloseAll(commandData) {
        return _closeList(MainViewManager.getAllOpenFiles(),
                                    (commandData && commandData.promptOnly), (commandData && commandData._forceClose));
    }
Closes all open files; equivalent to calling handleFileClose() for each document, except
that unsaved changes are confirmed once, in bulk.
@param {?{promptOnly: boolean, _forceClose: boolean}}
         If promptOnly is true, only displays the relevant confirmation UI and does NOT
         actually close any documents. This is useful when chaining close-all together with
         other user prompts that may be cancelable.
         If _forceClose is true, forces the files to close with no confirmation even if dirty.
         Should only be used for unit test cleanup.
@return {$.Promise} a promise that is resolved when all files are closed