Implement GBXML export

Description

Implement the possibility to export a BIM model as GBXML

Additional Information

Started in https://github.com/FreeCAD/FreeCAD/blob/master/src/Mod/Arch/importGBXML.py Downstream ticket: https://github.com/yorikvanhavre/BIM_Workbench/issues/6 Bountysource: https://www.bountysource.com/issues/65465675-implement-gbxml-export