Added the stub window.c. Now we can really start the new GTK+ backend.
Added the stub window.c. Now we can really start the new GTK+ backend.

mantyr/libui,ProtonMail/libui,tbodt/libui,tbodt/libui,ProtonMail/libui,mantyr/libui,sclukey/libui,sclukey/libui,ProtonMail/libui,mantyr/libui