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/system_information/src/main.rs

@@ -1,5 +1,5 @@
 use iced::widget::{button, column, container, text};
-use iced::{system, Command, Element, Length};
+use iced::{system, Command, Element};
 
 pub fn main() -> iced::Result {
     iced::program("System Information - Iced", Example::update, Example::view)
@@ -136,11 +136,6 @@ impl Example {
             }
         };
 
-        container(content)
-            .center_x()
-            .center_y()
-            .width(Length::Fill)
-            .height(Length::Fill)
-            .into()
+        container(content).center().into()
     }
 }