Bug 488437 - Offer mechanism to enable diagnostic overlays in TMD-API * improve updating by avoiding recomputing know cached values * keep child diagnostic in sorted tree for faster access * delay refresh until validation is finished Change-Id: Ic9b0ac7ce07f7f956a543b525a02d47f7a7a3ce1 Signed-off-by: Johannes Faltermeier <jfaltermeier@eclipsesource.com>