Fix gdispass script

When trying to start `gdispass` prior to this change I get the following output:

```pytb
Traceback (most recent call last):
  File "/usr/bin/gdispass", line 25, in <module>
    gui = GUI(Settings())
TypeError: __init__() takes exactly 3 arguments (2 given)
```

I didn't look very thoroughly at how the `GUI` class is instantiated using the `dispass gui` command, so I may be missing some subleties here.