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

api/cpp/cbindgen.rs

@@ -749,8 +749,7 @@ fn gen_platform(
         .with_include("slint_internal.h")
         .with_after_include(
             r"
-namespace slint::platform { struct WindowProperties; }
-namespace slint::cbindgen_private { using platform::WindowProperties; }
+namespace slint::cbindgen_private { struct WindowProperties; }
 ",
         )
         .generate()