gtk-demo/demowindow: Order #includes correctly
gtk-demo/demowindow: Order #includes correctly

in the spirit of
https://git.gnome.org/browse/gtk+/tree/docs/CODING-STYLE#n458

“Every file must include config.h first, then its own header, then other
GTK+ headers that it needs, then system and third-party headers that it
needs.”

For the “other GTK+ headers” part, I took the liberty of using the order
I normally follow, which is GLib headers followed by GIO ones followed
by GTK+ ones. I use this as it follows their order in the software stack

GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm