Move widgets from core to native and web

Also made fields private and improved `Renderer` traits.

web/src/widget/row.rs

@@ -1,8 +1,112 @@
-use crate::{Bus, Element, Widget};
+use crate::{Align, Bus, Element, Length, Widget};
 
 use dodrio::bumpalo;
+use std::u32;
 
-pub type Row<'a, Message> = iced_core::Row<Element<'a, Message>>;
+/// A container that distributes its contents horizontally.
+///
+/// A [`Row`] will try to fill the horizontal space of its container.
+///
+/// [`Row`]: struct.Row.html
+#[allow(missing_docs)]
+pub struct Row<'a, Message> {
+    spacing: u16,
+    padding: u16,
+    width: Length,
+    height: Length,
+    max_width: u32,
+    max_height: u32,
+    align_items: Align,
+    children: Vec<Element<'a, Message>>,
+}
+
+impl<'a, Message> Row<'a, Message> {
+    /// Creates an empty [`Row`].
+    ///
+    /// [`Row`]: struct.Row.html
+    pub fn new() -> Self {
+        Row {
+            spacing: 0,
+            padding: 0,
+            width: Length::Fill,
+            height: Length::Shrink,
+            max_width: u32::MAX,
+            max_height: u32::MAX,
+            align_items: Align::Start,
+            children: Vec::new(),
+        }
+    }
+
+    /// Sets the horizontal spacing _between_ elements.
+    ///
+    /// Custom margins per element do not exist in Iced. You should use this
+    /// method instead! While less flexible, it helps you keep spacing between
+    /// elements consistent.
+    pub fn spacing(mut self, units: u16) -> Self {
+        self.spacing = units;
+        self
+    }
+
+    /// Sets the padding of the [`Row`].
+    ///
+    /// [`Row`]: struct.Row.html
+    pub fn padding(mut self, units: u16) -> Self {
+        self.padding = units;
+        self
+    }
+
+    /// Sets the width of the [`Row`].
+    ///
+    /// [`Row`]: struct.Row.html
+    pub fn width(mut self, width: Length) -> Self {
+        self.width = width;
+        self
+    }
+
+    /// Sets the height of the [`Row`].
+    ///
+    /// [`Row`]: struct.Row.html
+    pub fn height(mut self, height: Length) -> Self {
+        self.height = height;
+        self
+    }
+
+    /// Sets the maximum width of the [`Row`].
+    ///
+    /// [`Row`]: struct.Row.html
+    pub fn max_width(mut self, max_width: u32) -> Self {
+        self.max_width = max_width;
+        self
+    }
+
+    /// Sets the maximum height of the [`Row`].
+    ///
+    /// [`Row`]: struct.Row.html
+    pub fn max_height(mut self, max_height: u32) -> Self {
+        self.max_height = max_height;
+        self
+    }
+
+    /// Sets the vertical alignment of the contents of the [`Row`] .
+    ///
+    /// [`Row`]: struct.Row.html
+    pub fn align_items(mut self, align: Align) -> Self {
+        self.align_items = align;
+        self
+    }
+
+    /// Adds an [`Element`] to the [`Row`].
+    ///
+    /// [`Element`]: ../struct.Element.html
+    /// [`Row`]: struct.Row.html
+    pub fn push<E>(mut self, child: E) -> Self
+    where
+        E: Into<Element<'a, Message>>,
+    {
+        self.children.push(child.into());
+        self
+    }
+}
 
 impl<'a, Message> Widget<Message> for Row<'a, Message> {
     fn node<'b>(