Refactor IfcAttributes to be called IfcData.
Refactor IfcAttributes to be called IfcData.

 * IfcAttributes as a name is misleading, as currently it stores GUID (yes, an attribute), but also stores IFC export settings (i.e. not an IFC "attribute", per say)
 * The previous description of "Custom IFC properties ..." is also misleading, as properties might suggest it stores property sets, which it doesn't (IfcProperties does, currently)
 * IfcData is more generic and allows us to store more data in it in the future

Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master