Draft virtual Button, Column, and Text

... as well as a very naive diffing strategy!

virtual/src/widget/element.rs

@@ -0,0 +1,21 @@
+use crate::Widget;
+
+pub struct Element<Message, Renderer> {
+    widget: Box<dyn Widget<Message, Renderer>>,
+}
+
+impl<Message, Renderer> Element<Message, Renderer> {
+    pub fn new(widget: impl Widget<Message, Renderer> + 'static) -> Self {
+        Self {
+            widget: Box::new(widget),
+        }
+    }
+
+    pub fn as_widget(&self) -> &dyn Widget<Message, Renderer> {
+        self.widget.as_ref()
+    }
+
+    pub fn as_widget_mut(&mut self) -> &mut dyn Widget<Message, Renderer> {
+        self.widget.as_mut()
+    }
+}