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/screenshot/src/main.rs

@@ -123,12 +123,10 @@ impl Example {
         };
 
         let image = container(image)
+            .center_y()
             .padding(10)
             .style(container::rounded_box)
-            .width(Length::FillPortion(2))
-            .height(Length::Fill)
-            .center_x()
-            .center_y();
+            .width(Length::FillPortion(2));
 
         let crop_origin_controls = row![
             text("X:")
@@ -213,12 +211,7 @@ impl Example {
             .spacing(40)
         };
 
-        let side_content = container(controls)
-            .align_x(alignment::Horizontal::Center)
-            .width(Length::FillPortion(1))
-            .height(Length::Fill)
-            .center_y()
-            .center_x();
+        let side_content = container(controls).center_y();
 
         let content = row![side_content, image]
             .spacing(10)
@@ -226,13 +219,7 @@ impl Example {
             .height(Length::Fill)
             .align_items(Alignment::Center);
 
-        container(content)
-            .width(Length::Fill)
-            .height(Length::Fill)
-            .padding(10)
-            .center_x()
-            .center_y()
-            .into()
+        container(content).padding(10).into()
     }
 
     fn subscription(&self) -> Subscription<Message> {