imagePage: Move popover out of LpImage

It cause 'not measured' errors and we have enough
things to handle in LpImage already.

Part-of: <https://gitlab.gnome.org/Incubator/loupe/-/merge_requests/146>

src/widgets/image.rs

@@ -269,10 +269,6 @@ mod imp {
                 .set_target(&adw::CallbackAnimationTarget::new(|_| {}));
             obj.zoom_animation()
                 .set_target(&adw::CallbackAnimationTarget::new(|_| {}));
-
-            while let Some(child) = obj.first_child() {
-                child.unparent();
-            }
         }
     }