Resolve "Load/Save SFG to file"

b_asic/GUI/about_window.py

b_asic/GUI/arrow.py

b_asic/GUI/drag_button.py

b_asic/GUI/gui_interface.py

b_asic/GUI/main_window.py

b_asic/GUI/port_button.py

b_asic/GUI/properties_window.py

b_asic/GUI/select_sfg_window.py

b_asic/GUI/show_pc_window.py

b_asic/GUI/simulate_sfg_window.py

b_asic/__init__.py

b_asic/operation.py

b_asic/save_load_structure.py

test/test_operation.py

test/test_sfg.py