Introduce center widget helper

... and also make `center_x` and `center_y` set
`width` and `height` to `Length::Fill`, respectively.

This targets the most common use case when centering
things and removes a bunch of boilerplate as a result.

examples/scrollable/src/main.rs

@@ -327,7 +327,7 @@ impl ScrollableDemo {
                 .spacing(10)
                 .into();
 
-        container(content).padding(20).center_x().center_y().into()
+        container(content).padding(20).into()
     }
 
     fn theme(&self) -> Theme {