function adjustDistanceFromRoot(oNode, iDiff) {
			oNode.DistanceFromRoot += iDiff;
			(mChildrenByParentId[oNode.ID] || [])
				.forEach((oChild) => { adjustDistanceFromRoot(oChild, iDiff); });
		}
Adjust the DistanceFromRoot of the given node (and all of its descendants) by the given
difference.

@param {object} oNode - A node
@param {number} iDiff - Some difference