Enable scripted and non-GUI workflow

[CsatiZoltan]

I love this workbench. However, currently, I have to copy-paste parts of it so that I can use it in my Python script. Yes, I could import it as a package, but it still cannot be used in non-GUI mode (i.e. starting from the FreeCADCmd executable). E.g. FreeCADGui.addCommand does not exist in non-GUI mode. This is a pity because I need to create classes that extract functionalities from the Curves workbench. It also hampers me from contributing to your workbench. Is there an incentive that Curves would be usable without the GUI?