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/websocket/src/main.rs

@@ -1,9 +1,7 @@
 mod echo;
 
 use iced::alignment::{self, Alignment};
-use iced::widget::{
-    button, column, container, row, scrollable, text, text_input,
-};
+use iced::widget::{button, center, column, row, scrollable, text, text_input};
 use iced::{color, Command, Element, Length, Subscription};
 use once_cell::sync::Lazy;
 
@@ -85,14 +83,10 @@ impl WebSocket {
 
     fn view(&self) -> Element<Message> {
         let message_log: Element<_> = if self.messages.is_empty() {
-            container(
+            center(
                 text("Your messages will appear here...")
                     .color(color!(0x888888)),
             )
-            .width(Length::Fill)
-            .height(Length::Fill)
-            .center_x()
-            .center_y()
             .into()
         } else {
             scrollable(