title: gdk_x11_drawable_get_xid alternative for GTK3
body: <p>Recently an app I use (Firefox) was upgraded to GTK3. My plugin relied on <code>gdk_x11_drawable_get_xid</code> as Firefox stores the <code>GDKWindow*</code> in some vars available to us developers. I make plugins.</p>
  
  <p>Is there a GTK3 way to go from <code>GDKWindow*</code> to <code>XID</code>?</p>
  
  <p>Thanks</p>