Get rid of abi::datastructures::WindowProperties

Instead, pass a reference to the root item when mapping the window,
at which point we can downcast to the new Window item. If we have one,
then we'll read its width/height (for initial values) and install
bindings to keep them up-to-date.

tools/viewer/main.rs

@@ -36,6 +36,6 @@ fn main() -> std::io::Result<()> {
     };
 
     let component = c.create();
-    component.window().run(component.borrow(), &component.window_properties());
+    component.window().run(component.borrow(), component.root_item());
     Ok(())
 }