Deprecate ui.centered

It didn't do what it advertised to do, and there is no simple way
of doing that right now.

crates/egui/src/layout.rs

@@ -229,22 +229,10 @@ impl Layout {
         }
     }
 
-    /// For when you want to add a single widget to a layout, and that widget
-    /// should be centered horizontally and vertically.
-    #[inline(always)]
-    pub fn centered(main_dir: Direction) -> Self {
-        Self {
-            main_dir,
-            main_wrap: false,
-            main_align: Align::Center,
-            main_justify: false,
-            cross_align: Align::Center,
-            cross_justify: false,
-        }
-    }
-
     /// For when you want to add a single widget to a layout, and that widget
     /// should use up all available space.
+    ///
+    /// Only one widget may be added to the inner `Ui`!
     #[inline(always)]
     pub fn centered_and_justified(main_dir: Direction) -> Self {
         Self {

crates/egui/src/ui.rs

@@ -2007,12 +2007,14 @@ impl Ui {
         InnerResponse::new(inner, self.interact(rect, child_ui.id, Sense::hover()))
     }
 
-    /// This will make the next added widget centered in the available space.
+    #[deprecated = "Use ui.vertical_centered instead"]
     pub fn centered<R>(&mut self, add_contents: impl FnOnce(&mut Self) -> R) -> InnerResponse<R> {
-        self.with_layout_dyn(Layout::centered(Direction::TopDown), Box::new(add_contents))
+        self.vertical_centered(add_contents)
     }
 
     /// This will make the next added widget centered and justified in the available space.
+    ///
+    /// Only one widget may be added to the inner `Ui`!
     pub fn centered_and_justified<R>(
         &mut self,
         add_contents: impl FnOnce(&mut Self) -> R,