Show window state preservation in the state saving example

On https://docs.elementary.io/develop/apis/state-saving#use-the-settings-object, instead of the switch example, it would be useful to show the Human Interface Guidelines recommended window state saving/restoration code as this is something every app is expected to implement and the GNOME guidelines (https://developer.gnome.org/SaveWindowState/documentation/tutorials/save-state.html) are not directly implementable.