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/button.rs

@@ -0,0 +1,64 @@
+use crate::element::{self, Element};
+use crate::Widget;
+
+pub struct Button<Message, Renderer> {
+    content: Element<Message, Renderer>,
+    on_press: Option<Message>,
+}
+
+impl<Message, Renderer> Button<Message, Renderer> {
+    pub fn new(
+        content: impl element::Descriptor<Message, Renderer> + 'static,
+    ) -> Self {
+        Button {
+            content: Element::new(content),
+            on_press: None,
+        }
+    }
+
+    pub fn on_press(mut self, on_press: Message) -> Self {
+        self.on_press = Some(on_press);
+        self
+    }
+}
+
+impl<Message, Renderer> element::Descriptor<Message, Renderer>
+    for Button<Message, Renderer>
+where
+    Message: 'static + Clone,
+    Renderer: 'static,
+{
+    fn tag(&self) -> std::any::TypeId {
+        std::any::TypeId::of::<Self>()
+    }
+
+    fn build(&self) -> Box<dyn Widget<Message, Renderer>> {
+        Box::new(State { is_pressed: false })
+    }
+
+    fn children(&self) -> &[Element<Message, Renderer>] {
+        std::slice::from_ref(&self.content)
+    }
+
+    fn clone(&self) -> Box<dyn element::Descriptor<Message, Renderer>> {
+        Box::new(Clone::clone(self))
+    }
+}
+
+impl<Message, Renderer> Clone for Button<Message, Renderer>
+where
+    Message: Clone,
+{
+    fn clone(&self) -> Self {
+        Self {
+            content: self.content.clone(),
+            on_press: self.on_press.clone(),
+        }
+    }
+}
+
+pub struct State {
+    is_pressed: bool,
+}
+
+impl<Message, Renderer> Widget<Message, Renderer> for State {}