application: Present active window if available

Without this, the application crashes on second activation:

thread 'main' panicked at src/application.rs:34:49:
called `Result::unwrap()` on an `Err` value: WeakRef(GWeakRef @ 0x6293601b02d0 { priv_: GWeakRef_priv @ 0x6293601b02d0 { p: 0x629360da2ba0 } }, PhantomData<*mut gnome_tour::widgets::window::Window>)