Media replacement && Mogrt Group fix

[phlexib]

First, this fixes a potential issue when a .Mogrt file has Groups. Originally, The applyEssentialValues.jsx script would not look recursively, and would miss properties inside groups.

Second, this branch allows for Media replacement in the Mogrt, using the prop.setAlternatesource method.
It also take care of transforming the file url one would use in Nexrender into a system path that would be understood by extendscript.

 if (prop.canSetAlternateSource) {
                var replacementItem = app.project.importFile(
                    new ImportOptions(new File(value))
                );
                prop.setAlternateSource(replacementItem);
            } else {
                prop.setValue(value);
            }

A Job Json with Media replacement would look like this:

{
  "template": {
    "src": "file:///path/to/file.mogrt",
    "composition": "Comp 1"
  },
  "actions": {
    "predownload": [
      {
        "module": "nexrender-action-mogrt-template",
        "essentialParameters": {
          "Intro_Text_Image": "file:///path/to/image.png",
          "Dog Name": "New Dog Name",
          "BG Color": [1, 1, 0, 1],
          "scale": [100, 100, 0]
        }
      }
    ]
  }
}

[vonstring]

By first glance this looks excellent! But would you mind separating changes fixing my formatting (which admittedly is all over the place) and actual changes into separate commits? It's quite hard to see what actual changes were made currently.