message: Added new progress dialog during saves
diff: <script src="https://cdn.jsdelivr.net/gh/gitbrent/pptxgenjs@latest/examples/images/base64Images.js" async></script>
  <!-- RELEASE (CDN)
  + -->
  <script src="https://cdn.jsdelivr.net/gh/gitbrent/pptxgenjs@latest/dist/pptxgen.bundle.js"></script>
  <script src="https://cdn.jsdelivr.net/gh/gitbrent/pptxgenjs@latest/demos/common/demos.js"></script>
  - -->
  <!--
  TODO: [only used during `-beta` dev cycles; update to below on release]
  <script src="https://cdn.jsdelivr.net/gh/gitbrent/pptxgenjs@master/dist/pptxgen.bundle.js"></script>
  <!-- LOCAL TESTING:
  <script src="../../libs/jszip.min.js"></script>
  <script src="../../dist/pptxgen.min.js"></script>
  - -->
  <script src="../../dist/pptxgen.bundle.js"></script>
  <script src="../common/demos.js"></script>
  + -->
  <script>
  // ==================================================================================================================
  function runAllDemos() {
  if (console.time) console.time('runAllDemos');
  + $('#modalBusy').modal('show');
  +
  runEveryTest()
  .catch(function(err) {
  console.error(err.toString());
  + $('#modalBusy').modal('hide');
  })
  .then(function() {
  if (console.timeEnd) console.timeEnd('runAllDemos');
  + $('#modalBusy').modal('hide');
  });
  }
  function execGenSlidesFunc(type) {
  if (console.time) console.time('execGenSlidesFunc: '+type);
  + $('#modalBusy').modal('show');
  +
  execGenSlidesFuncs(type)
  .catch(function(err) {
  console.error(err.toString());
  + $('#modalBusy').modal('hide');
  })
  .then(function() {
  if (console.timeEnd) console.timeEnd('execGenSlidesFunc: '+type);
  + $('#modalBusy').modal('hide');
  });
  }
  </script>
  </head>
  <body class="bg-light">
  + <div id="modalBusy" class="modal fade" tabindex="-1" role="dialog" data-backdrop="static" >
  + <div class="modal-dialog modal-dialog-centered" role="document">
  + <div class="modal-content">
  + <div class="modal-body">
  + <h3 class="text-primary">Please Wait</h3>
  + <div class="text-dark">Creating and saving presentation...</div>
  + </div>
  + <div class="modal-footer bg-light">
  + <div class="progress w-100" style="height:2rem;">
  + <div class="progress-bar progress-bar-striped progress-bar-animated w-100" role="progressbar" aria-valuenow="100" aria-valuemin="0" aria-valuemax="100"></div>
  + </div>
  + </div>
  + </div>
  + </div>
  + </div>
  +
  <nav class="navbar navbar-expand-lg bg-dark">
  <a class="navbar-brand" href="https://gitbrent.github.io/PptxGenJS/">
  <img src="images/favicon.png" width="30" height="30" class="d-inline-block align-top mr-2" alt="" />
  