Explicitely delete wx.App at the end of pronterface.py

Okai, this may not make that much sense. However when removing the mainloop,
pronterface was segfaulting when Python was freeing everything. As a
workaround, explictely destroy the wx application, which will cleanly free
things in order, while Python might destroy wx things in a random order.

pronterface.py

@@ -1527,3 +1527,4 @@ if __name__ == '__main__':
         app.MainLoop()
     except KeyboardInterrupt:
         pass
+    del app