Arch: misc improvements and bugfixes

* Allow mesh-based windows (with non-solid shape) to pass through
* Allow precast elements to be clones
* Structure elements now display a different icon when thry are clones
* Arch clones now get the same placements as their cloned object on create
* Fixed the bad export of clones to IFC
* Recoded the Site/Building/Floor IFC export to be more flexible

src/Mod/Arch/ArchStructure.py

@@ -504,6 +504,9 @@ class _ViewProviderStructure(ArchComponent.ViewProviderComponent):
 
     def getIcon(self):
         import Arch_rc
+        if hasattr(self,"Object"):
+            if self.Object.CloneOf:
+                return ":/icons/Arch_Structure_Clone.svg"
         return ":/icons/Arch_Structure_Tree.svg"
 
     def updateData(self,obj,prop):