Add platforms/qwindows.dll management

Scripts/developer_scripts/cgal_demo_copy_all_dlls_cygwin.sh

@@ -1,6 +1,7 @@
 #!/bin/bash
 
 #use this script from inside the build directory of the Polyhedron demo
+#Needs the Qt5_DIR env variable set to <Qt5_ROOT>/lib/cmake/Qt5
 
 declare config="Release"
 
@@ -33,5 +34,6 @@ for file in "${files[@]}"; do
     copy_dll "$dll" "$target_directory"
 
   done; #check dependencies
-    
 done #loop over directories
+mkdir -p "$target_directory/platforms"
+cp "$Qt5_DIR/../../../plugins/platforms/qwindows.dll" "$target_directory/platforms"