Core: Import STEP: Cancel button does not cancel (#16499)

* Fix importing .step file when user cancelled import settings dialog
* Refactor object loading python code, fix not using settings when STEP options dialog not shown
* Use custom exception type for user cancelling import instead of `RuntimeError`
* Pull python code out to external file

src/Base/PyObjectBase.cpp

@@ -49,6 +49,7 @@ PyObject* Base::PyExc_FC_ExpressionError = nullptr;
 PyObject* Base::PyExc_FC_ParserError = nullptr;
 PyObject* Base::PyExc_FC_CADKernelError = nullptr;
 PyObject* Base::PyExc_FC_PropertyError = nullptr;
+PyObject* Base::PyExc_FC_AbortIOException = nullptr;
 
 typedef struct {            //NOLINT
     PyObject_HEAD