Move SvgSelectors to new refinedBy prop

Unfotunately, I made a mistake interpreting the W3C model. To correct thist, the annotation selector now has a new prop refinedBy that should hold the SvgSelecto as a refinement of the previous temporal FragmentSelector.

The prop works just like the selector on target (it's an array as well), only that there is not another refinedBy prop on it.

https://www.w3.org/TR/annotation-model/#refinement-of-selection

Now a selector using SVG would now look like this:

{
  target: {
    id: TARGET_ID,
    selector: [{
      type: 'FragmentSelector',
      value: FRAGMENT_VALUE,
      conformsTo: 'http://www.w3.org/TR/media-frags/',
      refinedBy: [{
        type: 'SvgSelector',
        value: SVG_VALUE
      }]
    }]
  }
}