Add image references to the manifest

[HadrienGardeur]

With the on-going work on #180, we're now capable of detecting where an image is used, if it's decorative and which description (if any) is contained in the text.

This will be handled using a new references object in properties that can contain 1-n objects where:

• href links to the resources and can contain a CSS selector as a fragment (hello.xhtml#css(body%20%3E%20span:nth-child(16)))
• description contains the image description (including blank ones, which is different from a missing description)
• and role where we mostly look for presentation

{
  "href": "OPS/images/Moby-Dick_FE_title_page.jpg",
  "type": "image/jpeg",
  "size": 105155,
  "width": 840,
  "height": 1380,
  "properties": {
    "animated": false,
    "hash": [
      {
        "algorithm": "sha256",
        "value": "dJw6jaDD0O6tKBw29unMf1DxChpBiuWWbouTBZS+UlE="
      }
    ],
    "references": [
     {
       "href": "hello.xhtml#css(body%20%3E%20span:nth-child(16))",
       "description": "This is the description of the image",
       "role": ["presentation"]
     }
    ]
  }
}