Move widgets from core to native and web

Also made fields private and improved `Renderer` traits.

core/src/align.rs

@@ -16,3 +16,29 @@ pub enum Align {
     /// Align at the end of the cross axis.
     End,
 }
+
+/// The horizontal alignment of some resource.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum HorizontalAlignment {
+    /// Align left
+    Left,
+
+    /// Horizontally centered
+    Center,
+
+    /// Align right
+    Right,
+}
+
+/// The vertical alignment of some resource.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum VerticalAlignment {
+    /// Align top
+    Top,
+
+    /// Vertically centered
+    Center,
+
+    /// Align bottom
+    Bottom,
+}