window: Don't store window to avoid ref cycles

Use root() instead and move one constructor to on root for having the
root available.

src/widgets/image_window/actions.rs

@@ -22,6 +22,7 @@ use gtk::prelude::*;
 use strum::IntoEnumIterator;
 
 use crate::deps::*;
+use crate::util::root::ParentWindow;
 use crate::util::{Direction, Position};
 
 #[derive(strum::Display, strum::AsRefStr, strum::EnumIter)]