Implement scroll_by operation for scrollable

`scroll_by` allows scrolling an absolute offset
that is applied to the current scrolling position.

widget/src/container.rs

@@ -459,6 +459,7 @@ pub fn visible_bounds(id: Id) -> Task<Option<Rectangle>> {
             _state: &mut dyn widget::operation::Scrollable,
             _id: Option<&widget::Id>,
             bounds: Rectangle,
+            _content_bounds: Rectangle,
             translation: Vector,
         ) {
             match self.scrollables.last() {