Added Hull and Minkowski GUI commands
move core function of importCSG.processDXF

to OpenSCAD2Dgeom
Partly Revert "Replace BRepTools_ReShape with BRepBuilderAPI_MakeSolid in refinement algorithm"

This partly reverts commit 48f8d67b509a61875ee32fb809c8a66c4cafe6fe.

This reenables the workaround for the behavior of the Part Module.
As the changes in  src/Mod/Part/App/TopoShape.cpp and modelRefine.cpp
won't be backported to 0.13.