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/pokedex/src/main.rs

@@ -1,5 +1,5 @@
 use iced::futures;
-use iced::widget::{self, column, container, image, row, text};
+use iced::widget::{self, center, column, image, row, text};
 use iced::{Alignment, Command, Element, Length};
 
 pub fn main() -> iced::Result {
@@ -83,12 +83,7 @@ impl Pokedex {
             .align_items(Alignment::End),
         };
 
-        container(content)
-            .width(Length::Fill)
-            .height(Length::Fill)
-            .center_x()
-            .center_y()
-            .into()
+        center(content).into()
     }
 }