Improve Arrow.remove()

Currently, the only use of SFGMainWindow._ports is in Arrow.remove. It should be possible to rewrite this to not rely on SFGMainWindow._ports.

All DragButtons are available as SFGMainWindow._drag_buttons.values() and the ports associated with a DragButton as DragButton.port_list.

Also, the code looks quite complicated... (Although currently not will full code coverage.)

added Code quality SFG GUI labels

changed title from Improve {--}Arrow.remove(){--} to Improve Arrow.remove()

changed milestone to %Autumn version