Support for scrub bar with thumbnail images

Created by: TobbeEdgeware

Following the discussion at the dash.js biweekly meeting on March7, here is a request about implementing the low-complexity thumbnail solution that has been discussed in https://github.com/Dash-Industry-Forum/DASH-IF-IOP/issues/119

The proposed design is out for community review at http://dashif.org/wp-content/uploads/2017/03/CR-image-thumbnails-v0.9.pdf