Client: Expose default input device's serial through integration

Default input device is not so useful
without serial for calls like xdg_toplevel_show_window_menu

This is done in the same way as
apptime/appusertime/gettimestamp in xcb's native interface.

Pick-to: 6.4
Change-Id: I20e7dee19cf20e819a557e23c539066945cecb46
Reviewed-by: David Edmundson <davidedmundson@kde.org>

src/plugins/platforms/wayland/qwaylandnativeinterface.cpp

@@ -98,6 +98,8 @@ void *QWaylandNativeInterface::nativeResourceForIntegration(const QByteArray &re
             return touch->wl_touch();
         return nullptr;
     }
+    if (lowerCaseResource == "serial")
+        return reinterpret_cast<void *>(quintptr(m_integration->display()->defaultInputDevice()->serial()));
 
     return nullptr;
 }