chore: re-export iced row and column

This removes the custom row and column implementations and uses the iced ones directly.

examples/about/src/main.rs

@@ -132,7 +132,7 @@ impl cosmic::Application for App {
     fn view(&self) -> Element<'_, Self::Message> {
         let show_about_button = widget::button::text("Show about").on_press(Message::ToggleAbout);
         let centered = cosmic::widget::container(
-            widget::column()
+            widget::column::with_capacity(1)
                 .push(show_about_button)
                 .width(Length::Fill)
                 .height(Length::Shrink)