MainWindow: Use html.escape. Fix Python 3.8.

Python 3.2 deprecated and 3.8 removes the escape function from the cgi module. The official documentation states to use html.escape as replacement.

This makes Alacarte work with Python 3 for me.

See: https://docs.python.org/3.7/library/cgi.html#cgi.escape

Btw, are you planing to tag a release with Python 3? The last was tagged 5 years ago.