Quit is a built-in class, because is very common. There are also Yes, No, Ok and Cancel builtins. You could do the same writing ['Quit'] and binding it a function that quits the program.

Binding is done using QT's signals/slots mechanism under the hood, and can be done in several ways:

- using the 'with' statement as in the example, but this is often limited

- using an events layer, that is, a replica of the GUI layout, with a callable for each widget that you want to bind.

- using the @gui.auto decorator on a function: it will try to guess.

All this is described in the tutorial: https://guietta.readthedocs.io/en/latest/tutorial.html , chapter "GUI actions"