Draft iced_virtual subcrate

The idea here is to expose a set of "virtual widgets" that can be used with a
`Virtual` widget and its `virtual::State`.

A virtual widget is a widget that does not contain any state, but instead is a
"virtual" representation of the "real" widget. The real widgets are stored in
the `virtual::State`.

Every time a new virtual widget tree is created during `view`, it is compared to
the previous one and "real" widgets are added / removed to the `virtual::State`.

Effectively, this removes the need to keep track of local widget state in the
application state and allows `view` to take an immutable reference to `self`.

To summarize, using this crate should allow users to remove `State` structs
in their application state.

Eventually, the strategy used here may be adopted generally and, as a result,
all of the widgets in `iced_native` would be replaced!

virtual/src/widget.rs

@@ -0,0 +1,52 @@
+mod button;
+
+pub use button::Button;
+
+use iced_native::layout::{self, Layout};
+use iced_native::renderer;
+use iced_native::{Hasher, Length, Point, Rectangle};
+
+pub trait Widget<Message, Renderer> {}
+
+pub(crate) enum Tree<Message, Renderer> {
+    Node {
+        widget: Box<dyn Widget<Message, Renderer>>,
+        children: Vec<Tree<Message, Renderer>>,
+    },
+    Leaf {
+        widget: Box<dyn Widget<Message, Renderer>>,
+    },
+}
+
+impl<Message, Renderer> Tree<Message, Renderer> {
+    pub fn width(&self) -> Length {
+        unimplemented! {}
+    }
+
+    pub fn height(&self) -> Length {
+        unimplemented! {}
+    }
+
+    pub fn hash_layout(&self, state: &mut Hasher) {
+        unimplemented! {}
+    }
+
+    pub fn layout(
+        &self,
+        renderer: &Renderer,
+        limits: &layout::Limits,
+    ) -> layout::Node {
+        unimplemented! {}
+    }
+
+    pub fn draw(
+        &self,
+        renderer: &mut Renderer,
+        style: &renderer::Style,
+        layout: Layout<'_>,
+        cursor_position: Point,
+        viewport: &Rectangle,
+    ) {
+        unimplemented! {}
+    }
+}