Picture highlighting

[benel]

Find a way to highlight part of pictures.

The tagging would be then similar to what is done with text fragments.

Note:

• Porphyry already knows how to compute picture fragments co-occurrence (see code),
• Steatite already knows how to create a thumbnail from a picture fragment (see code).

[benel]

When a picture resource is loaded in Firefox, it seems that an HTML stub is automatically created.

We can hope this is possible with our extension to change the DOM to include SVG rectangles for stored fragments and the current selection.