Add the Publication's positionList

[mickael-menu]

Fixes readium/r2-testapp-kotlin#291

• Add the Publication's positionList (see Calculating the Publication.positionList architecture#101).
• Refactor and unit test the Locator model.
  • This requires some changes in the testapp, see the migration guide here.
  • Add the Locator.Locations.totalProgression
  • Following the lead from the Publication model, the HTML extension is implemented dynamically using a Locator.Locations.otherLocations JSON object.

[mickael-menu]

This PR (and the ones associated) are ready for review.