FIX: Correct deletion of matplotlib exporter window object (#868)

E.g. when opening the Matplotlib exporter multiple times, and one closes one instance, Python crashes.
This is caused by the Matplotlib QMainWindow listening to the closeEvent and deleting the only reference of
the window before it is closed properly.

pyqtgraph/exporters/Matplotlib.py

@@ -124,5 +124,4 @@ class MatplotlibWindow(QtGui.QMainWindow):
         
     def closeEvent(self, ev):
         MatplotlibExporter.windows.remove(self)
-
-
+        self.deleteLater()