0022795: Make possible to display some presentable objects in overlay of others, groupped by display priority

src/SelectMgr/SelectMgr_SelectableObject.cdl

@@ -26,6 +26,7 @@ uses
     SequenceOfSelection   from SelectMgr,
     TypeOfPresentation3d  from PrsMgr,
     Presentation          from Prs3d,
+    PresentationManager   from PrsMgr,
     PresentationManager3d from PrsMgr,
     SequenceOfOwner       from SelectMgr,
     NameOfColor           from Quantity,
@@ -169,7 +170,17 @@ is
     
     GetSelectPresentation(   me: mutable;
     	    	    	     TheMgr:  PresentationManager3d from PrsMgr )  returns Presentation from Prs3d  is  static; 
-			      
+	
+   SetZLayer ( me         : mutable;
+               thePrsMgr  : PresentationManager from PrsMgr;
+               theLayerId : Integer from Standard )
+     is redefined virtual;
+   ---Purpose: Set Z layer ID and update all presentations of
+   -- the selectable object. The layer can be set only for displayed object.
+   -- If all object presentations are removed, the layer ID will be set to 
+   -- default value when computing presentation. The layers mechanism allows 
+   -- drawing objects in higher layers in overlay of objects in lower layers.
+
 fields
 
     myselections        : SequenceOfSelection is protected;