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/Resources/Arch.qrc

@@ -21,6 +21,7 @@
         <file>icons/Arch_SectionPlane_Tree.svg</file>
         <file>icons/Arch_Site_Tree.svg</file>
         <file>icons/Arch_Structure_Tree.svg</file>
+        <file>icons/Arch_Structure_Clone.svg</file>
         <file>icons/Arch_Window_Tree.svg</file>
         <file>icons/Arch_Axis.svg</file>
         <file>icons/Arch_Axis_Tree.svg</file>