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/editor/src/main.rs

@@ -277,7 +277,7 @@ fn action<'a, Message: Clone + 'a>(
     label: &'a str,
     on_press: Option<Message>,
 ) -> Element<'a, Message> {
-    let action = button(container(content).width(30).center_x());
+    let action = button(container(content).center_x().width(30));
 
     if let Some(on_press) = on_press {
         tooltip(