Resolve "Load/Save SFG to file"

Save and load sfg to file, preserves positions when loading the sfg into the gui. Also minor fixes to connection etc.

This can't be merged until the __str__ is fixed for some sfgs. Closes #66 (closed)

b_asic/GUI/arrow.py

@@ -31,7 +31,6 @@ class Arrow(QGraphicsLineItem):
         self.signal.remove_source()
         self._window.scene.removeItem(self)
         self._window.signalList.remove(self)
-        del self._window.signalPortDict[self]
 
     def moveLine(self):
         self.setPen(QPen(Qt.black, 3))