The pad controller owns a reference to the window (as an action group)
and the window (as a widget) owns a reference to the pad controller.
This is a reference cycle.

Usually, reference cycles are resolved in dispose(), which can get
called multiple times. However, GTK removes the controllers during
finalize(). We end up calling finalize() recursively, which is a big
problem.

So our only option is to manually remove the controller before starting
the destruction of the window.