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.

examples/gallery/gallery.60

@@ -1,6 +1,6 @@
 import { Button, CheckBox, SpinBox, Slider } from "sixtyfps_widgets.60";
 
-App := Rectangle {
+App := Window {
     GridLayout {
         spacing: 10lx;
         Row {

examples/printerdemo/ui/printerdemo.60

@@ -115,7 +115,7 @@ SettingsPage := Page {
     }
 }
 
-MainWindow := Rectangle {
+MainWindow := Window {
 
     width: 800lx;
     height: 600lx;