Add g_queue_clear_full() API

g_queue_clear() currently directs people to manually free all the elements in the queue before calling it. It would be more convenient (and more efficient: O(N) instead of O(2N)) if there was a g_queue_clear_full() function which look a GDestroyNotify to do that for you. This would match the other *_clear_full() APIs in GLib.

This could at least be used inside D-Bus; see https://bugs.freedesktop.org/page.cgi?id=splinter.html&bug=107349&attachment=140790. I’m sure there would be other users.