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/lib.rs

@@ -0,0 +1,61 @@
+mod element;
+pub mod widget;
+
+pub use element::Element;
+pub use widget::Widget;
+
+use iced_native::layout::{self, Layout};
+use iced_native::renderer;
+use iced_native::{Hasher, Length, Point, Rectangle};
+
+pub struct Virtual<'a, Message, Renderer> {
+    state: &'a mut State<Message, Renderer>,
+}
+
+pub struct State<Message, Renderer> {
+    widget_tree: widget::Tree<Message, Renderer>,
+    last_element: Element<Message, Renderer>,
+}
+
+impl<'a, Message, Renderer> iced_native::Widget<Message, Renderer>
+    for Virtual<'a, Message, Renderer>
+where
+    Renderer: iced_native::Renderer,
+{
+    fn width(&self) -> Length {
+        self.state.widget_tree.width()
+    }
+
+    fn height(&self) -> Length {
+        self.state.widget_tree.height()
+    }
+
+    fn layout(
+        &self,
+        renderer: &Renderer,
+        limits: &layout::Limits,
+    ) -> layout::Node {
+        self.state.widget_tree.layout(renderer, limits)
+    }
+
+    fn hash_layout(&self, state: &mut Hasher) {
+        self.state.widget_tree.hash_layout(state)
+    }
+
+    fn draw(
+        &self,
+        renderer: &mut Renderer,
+        style: &renderer::Style,
+        layout: Layout<'_>,
+        cursor_position: Point,
+        viewport: &Rectangle,
+    ) {
+        self.state.widget_tree.draw(
+            renderer,
+            style,
+            layout,
+            cursor_position,
+            viewport,
+        )
+    }
+}