Route the ScrollBar's wheel event to the ScrollableWidget so it can handle it itself. This allows it to handle it consistently (e.g. speed) when the cursor is hovering the scroll bars rather than the widget's contents. Fixes #5419
Fixes edge cases in which hidden scrollbars could become visible and obscure content or fail to hide when no longer needed
Fixes vertical and horizontal overscroll in widgets that allow unnecessary scrollbars to be hidden.