Make iced_core::Button customizable

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

core/src/background.rs

@@ -0,0 +1,7 @@
+use crate::Color;
+
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum Background {
+    Color(Color),
+    // TODO: Add gradient and image variants
+}