card layout editing font is a bit large

https://forums.ankiweb.net/t/anki-2-1-50-inconvenient-interface-size/19024

clayout.py has the following line:

```python
editor.setFont(QFont("Courier"))
```

Attempting to set a point size of the font prior to calling setFont() does not work; perhaps Courier is not available in the smaller size, or not available at all? We may need a different font or approach on Windows.