[ArchStairs] Fix-hideSubobjects

Arch Objects usually has hideSubobjects to hide children.

ArchStairs does not have this and children (e.g. Additions, Subtraction [Railing to be added]) does not hide; this fix the problem.

Other Associated Inconsistency
- Other movable childre and visibility behaviour (like Railing of Stairs should be children) as discussed in https://forum.freecad.org/viewtopic.php?p=844965#p844920 would be added in subsequent PR

src/Mod/BIM/ArchStairs.py

@@ -597,6 +597,7 @@ class _Stairs(ArchComponent.Component):
 
     def onChanged(self,obj,prop):
 
+        self.hideSubobjects(obj,prop)
         ArchComponent.Component.onChanged(self,obj,prop)
 
         if (prop == "ArchSketchPropertySet"