C++: Api changes in the WindowAdapter

- s/physical_size/size/ (consistant with the slint::Window API)
 - remove const of virtual function  (they don't need to be const and
   make implementation potentially easier)
 - Move the WindowProperties in it

examples/cpp/platform_native/windowadapter_win.h

@@ -62,7 +62,7 @@ struct MyWindowAdapter : public slint::platform::WindowAdapter
 
     slint::platform::AbstractRenderer &renderer() override { return m_renderer.value(); }
 
-    slint::PhysicalSize physical_size() const override
+    slint::PhysicalSize size() override
     {
         RECT r;
         GetWindowRect(hwnd, &r);