Make iced_core::Button customizable

Now it supports:
  - Any kind of content
  - Custom border radius
  - Custom background

native/src/widget/column.rs

@@ -11,7 +11,7 @@ impl<'a, Message, Renderer> Widget<Message, Renderer>
 where
     Renderer: self::Renderer,
 {
-    fn node(&self, renderer: &mut Renderer) -> Node {
+    fn node(&self, renderer: &Renderer) -> Node {
         let mut children: Vec<Node> = self
             .children
             .iter()