Move widgets from core to native and web

Also made fields private and improved `Renderer` traits.

web/src/widget/checkbox.rs

@@ -1,8 +1,62 @@
-use crate::{Bus, Element, Widget};
+use crate::{Bus, Color, Element, Widget};
 
 use dodrio::bumpalo;
 
-pub use iced_core::Checkbox;
+/// A box that can be checked.
+///
+/// # Example
+///
+/// ```
+/// # use iced_web::Checkbox;
+///
+/// pub enum Message {
+///     CheckboxToggled(bool),
+/// }
+///
+/// let is_checked = true;
+///
+/// Checkbox::new(is_checked, "Toggle me!", Message::CheckboxToggled);
+/// ```
+///
+/// ![Checkbox drawn by Coffee's renderer](https://github.com/hecrj/coffee/blob/bda9818f823dfcb8a7ad0ff4940b4d4b387b5208/images/ui/checkbox.png?raw=true)
+pub struct Checkbox<Message> {
+    is_checked: bool,
+    on_toggle: Box<dyn Fn(bool) -> Message>,
+    label: String,
+    label_color: Option<Color>,
+}
+
+impl<Message> Checkbox<Message> {
+    /// Creates a new [`Checkbox`].
+    ///
+    /// It expects:
+    ///   * a boolean describing whether the [`Checkbox`] is checked or not
+    ///   * the label of the [`Checkbox`]
+    ///   * a function that will be called when the [`Checkbox`] is toggled. It
+    ///     will receive the new state of the [`Checkbox`] and must produce a
+    ///     `Message`.
+    ///
+    /// [`Checkbox`]: struct.Checkbox.html
+    pub fn new<F>(is_checked: bool, label: &str, f: F) -> Self
+    where
+        F: 'static + Fn(bool) -> Message,
+    {
+        Checkbox {
+            is_checked,
+            on_toggle: Box::new(f),
+            label: String::from(label),
+            label_color: None,
+        }
+    }
+
+    /// Sets the color of the label of the [`Checkbox`].
+    ///
+    /// [`Checkbox`]: struct.Checkbox.html
+    pub fn label_color<C: Into<Color>>(mut self, color: C) -> Self {
+        self.label_color = Some(color.into());
+        self
+    }
+}
 
 impl<Message> Widget<Message> for Checkbox<Message>
 where