Reduce WindowAdapter API slightly (#4304)

Move the set_fullscreen function added to the WindowAdapter trait in 779aff0b39
to be a function in WindowProperties instead.
That way it'll be easier in the future to extend this with other window states without
having to modify or break the WindowAdapter trait API.

api/cpp/include/slint-platform.h

@@ -243,6 +243,12 @@ public:
             return out;
         }
 
+        /// Returns true if the window should be shown fullscreen; false otherwise.
+        bool fullscreen() const
+        {
+            return cbindgen_private::slint_window_properties_get_fullscreen(inner());
+        }
+
         /// This struct describes the layout constraints of a window.
         ///
         /// It is the return value of WindowProperties::layout_constraints().

api/cpp/platform.rs

@@ -111,6 +111,11 @@ pub extern "C" fn slint_window_properties_get_background(
     *out = wp.background();
 }
 
+#[no_mangle]
+pub extern "C" fn slint_window_properties_get_fullscreen(wp: &WindowProperties) -> bool {
+    wp.fullscreen()
+}
+
 #[repr(C)]
 #[derive(Clone, Copy)]
 /// a Repr(C) variant of slint::platform::LayoutConstraints