Move widgets from core to native and web

Also made fields private and improved `Renderer` traits.

web/src/widget/slider.rs

@@ -1,8 +1,55 @@
-use crate::{Bus, Element, Widget};
+use crate::{Bus, Element, Length, Widget};
 
 use dodrio::bumpalo;
+use std::{ops::RangeInclusive, rc::Rc};
 
-pub use iced_core::slider::*;
+pub struct Slider<'a, Message> {
+    _state: &'a mut State,
+    range: RangeInclusive<f32>,
+    value: f32,
+    on_change: Rc<Box<dyn Fn(f32) -> Message>>,
+    width: Length,
+}
+
+impl<'a, Message> Slider<'a, Message> {
+    /// Creates a new [`Slider`].
+    ///
+    /// It expects:
+    ///   * the local [`State`] of the [`Slider`]
+    ///   * an inclusive range of possible values
+    ///   * the current value of the [`Slider`]
+    ///   * a function that will be called when the [`Slider`] is dragged.
+    ///   It receives the new value of the [`Slider`] and must produce a
+    ///   `Message`.
+    ///
+    /// [`Slider`]: struct.Slider.html
+    /// [`State`]: struct.State.html
+    pub fn new<F>(
+        state: &'a mut State,
+        range: RangeInclusive<f32>,
+        value: f32,
+        on_change: F,
+    ) -> Self
+    where
+        F: 'static + Fn(f32) -> Message,
+    {
+        Slider {
+            _state: state,
+            value: value.max(*range.start()).min(*range.end()),
+            range,
+            on_change: Rc::new(Box::new(on_change)),
+            width: Length::Fill,
+        }
+    }
+
+    /// Sets the width of the [`Slider`].
+    ///
+    /// [`Slider`]: struct.Slider.html
+    pub fn width(mut self, width: Length) -> Self {
+        self.width = width;
+        self
+    }
+}
 
 impl<'a, Message> Widget<Message> for Slider<'a, Message>
 where
@@ -60,3 +107,11 @@ where
         Element::new(slider)
     }
 }
+
+pub struct State;
+
+impl State {
+    pub fn new() -> Self {
+        Self
+    }
+}