g_source_is_destroyed example uses deprecated GDK API
When using the example from https://developer.gnome.org/glib/stable/glib-The-Main-Event-Loop.html#g-source-is-destroyed the build complains about using deprecated
gdk_threads_leave. Reading https://developer.gnome.org/gdk3/stable/gdk3-Threads.html#gdk3-Threads.description, those functions might not even be necessary as, in my understanding,
g_idle_add will run the callback in the main thread, which is also the UI thread.