function findSelectedThumbnails(thumbnailGrid) {
  var thumbs = thumbnailGrid.widgets();
  var ids = [];
  thumbs.forEach(function f(e) {
    var id = e.widgets().get(0).getValue();
    var checked = e.widgets().get(2).getValue();
    if (checked) {
      ids.push(COLLECTION_ID + '/' + id);
    }
  });
  return ids;
}
Given a thumbnailGrid widget, determines which thumbnails have been selected
by the user to include in a new mosaic.