Move widgets from core to native and web

Also made fields private and improved `Renderer` traits.

native/src/widget/container.rs

@@ -1,10 +1,94 @@
 use std::hash::Hash;
 
-use crate::{layout, Element, Event, Hasher, Layout, Length, Point, Widget};
+use crate::{
+    layout, Align, Element, Event, Hasher, Layout, Length, Point, Widget,
+};
 
-/// A container that distributes its contents vertically.
-pub type Container<'a, Message, Renderer> =
-    iced_core::Container<Element<'a, Message, Renderer>>;
+use std::u32;
+
+/// An element decorating some content.
+///
+/// It is normally used for alignment purposes.
+#[allow(missing_docs)]
+pub struct Container<'a, Message, Renderer> {
+    width: Length,
+    height: Length,
+    max_width: u32,
+    max_height: u32,
+    horizontal_alignment: Align,
+    vertical_alignment: Align,
+    content: Element<'a, Message, Renderer>,
+}
+
+impl<'a, Message, Renderer> Container<'a, Message, Renderer> {
+    /// Creates an empty [`Container`].
+    ///
+    /// [`Container`]: struct.Container.html
+    pub fn new<T>(content: T) -> Self
+    where
+        T: Into<Element<'a, Message, Renderer>>,
+    {
+        Container {
+            width: Length::Shrink,
+            height: Length::Shrink,
+            max_width: u32::MAX,
+            max_height: u32::MAX,
+            horizontal_alignment: Align::Start,
+            vertical_alignment: Align::Start,
+            content: content.into(),
+        }
+    }
+
+    /// Sets the width of the [`Container`].
+    ///
+    /// [`Container`]: struct.Container.html
+    pub fn width(mut self, width: Length) -> Self {
+        self.width = width;
+        self
+    }
+
+    /// Sets the height of the [`Container`].
+    ///
+    /// [`Container`]: struct.Container.html
+    pub fn height(mut self, height: Length) -> Self {
+        self.height = height;
+        self
+    }
+
+    /// Sets the maximum width of the [`Container`].
+    ///
+    /// [`Container`]: struct.Container.html
+    pub fn max_width(mut self, max_width: u32) -> Self {
+        self.max_width = max_width;
+        self
+    }
+
+    /// Sets the maximum height of the [`Container`] in pixels.
+    ///
+    /// [`Container`]: struct.Container.html
+    pub fn max_height(mut self, max_height: u32) -> Self {
+        self.max_height = max_height;
+        self
+    }
+
+    /// Centers the contents in the horizontal axis of the [`Container`].
+    ///
+    /// [`Container`]: struct.Container.html
+    pub fn center_x(mut self) -> Self {
+        self.horizontal_alignment = Align::Center;
+
+        self
+    }
+
+    /// Centers the contents in the vertical axis of the [`Container`].
+    ///
+    /// [`Container`]: struct.Container.html
+    pub fn center_y(mut self) -> Self {
+        self.vertical_alignment = Align::Center;
+
+        self
+    }
+}
 
 impl<'a, Message, Renderer> Widget<Message, Renderer>
     for Container<'a, Message, Renderer>