Implement Container widget

Remove `align_self` and `justify_content` methods

core/src/widget/container.rs

@@ -0,0 +1,94 @@
+use crate::{Align, Length};
+
+use std::u32;
+
+#[derive(Debug)]
+pub struct Container<Element> {
+    pub width: Length,
+    pub height: Length,
+    pub max_width: u32,
+    pub max_height: u32,
+    pub padding: u16,
+    pub horizontal_alignment: Align,
+    pub vertical_alignment: Align,
+    pub content: Element,
+}
+
+impl<Element> Container<Element> {
+    /// Creates an empty [`Container`].
+    ///
+    /// [`Container`]: struct.Container.html
+    pub fn new<T>(content: T) -> Self
+    where
+        T: Into<Element>,
+    {
+        Container {
+            width: Length::Shrink,
+            height: Length::Shrink,
+            max_width: u32::MAX,
+            max_height: u32::MAX,
+            horizontal_alignment: Align::Start,
+            vertical_alignment: Align::Start,
+            padding: 0,
+            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
+    }
+
+    /// Sets the padding of the [`Container`].
+    ///
+    /// [`Container`]: struct.Container.html
+    pub fn padding(mut self, units: u16) -> Self {
+        self.padding = units;
+        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
+    }
+}