outline-modification

Adds a private API to insert an outline

 unique_ptr<PDFDoc> doc = PDFDocFactory().createPDFDoc(GooString(argv[1]));
 Outline * outline = doc->getOutline();
 outline->insertOutline({
                           { "Preface", 17, {} },
                           { "1.  Introduction", 20, {
                                                       { "1.1 Subsection", 22, {} },
                                                     } },
                           { "2.  Principles of Steady-State Converter Analysis", 31, {} },
                          { "3.  Steady-State Equivalent Circuit Modeling, Losses, and Efficiency", 56, {}},
                          { "4.  Switch Realization", 79, {} },
                          });
  GooString outfilename("output.pdf");
  doc->saveAs(&outfilename);