function setFileViewFocus(fileSelectionFocus) {
        if (fileSelectionFocus !== PROJECT_MANAGER && fileSelectionFocus !== WORKING_SET_VIEW) {
            console.error("Bad parameter passed to FileViewController.setFileViewFocus");
            return;
        }

        if (_fileSelectionFocus !== fileSelectionFocus) {
            _fileSelectionFocus = fileSelectionFocus;
            exports.trigger("fileViewFocusChange");
        }
    }
Modifies the selection focus in the project side bar. A file can either be selected
in the working set (the open files) or in the file tree, but not both.
@param {String} fileSelectionFocus - either PROJECT_MANAGER or WORKING_SET_VIEW