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/Draft/Draft.py

@@ -2477,6 +2477,7 @@ def clone(obj,delta=None):
         base = getCloneBase(obj[0])
         cl.Label = prefix + base.Label
         cl.CloneOf = base
+        cl.Placement = obj[0].Placement
         return cl
     else:
         cl = FreeCAD.ActiveDocument.addObject("Part::AttachableObjectPython","Clone")