0030668: Visualization - revise adding ZLayer API

0030668: Visualization - revise adding ZLayer API

V3d_Viewer::AddZLayer() has been replaced by
V3d_Viewer::InsertLayerBefore() and V3d_Viewer::InsertLayerAfter() methods
allowing to specify Layer settings and index of existing layer to insert new one before/after.
The behavior of V3d_Viewer::AddZLayer() has been adjusted to append
new Layer before Graphic3d_ZLayerId_Top
(between Graphic3d_ZLayerId_Default and Graphic3d_ZLayerId_Top).

Graphic3d_Layer now provides LayerId() property.
Graphic3d_ZLayerSettings has been extended with IsRaytracable() property;
Ray-Tracing now processes multiple layers instead of Graphic3d_ZLayerId_Default.

Moved some methods from OpenGl_View to the base class Graphic3d_CView.