gdkwindow: Store children list nodes in GdkWindow structure

gdkwindow: Store children list nodes in GdkWindow structure

This avoids a bunch of allocations, and additionally it has better
cache behaviour, as we don't follow pointers to the separate GList
node memory areas during traversal.

From Christian Hergert:

  This machine is a Retina mac book pro so I've been working on getting
  GtkTextView (GtkPixelCache) up to our performance level on
  X11/Wayland. I'm seeing a jump from about 43 FPS to about 50 FPS.

https://bugzilla.gnome.org/show_bug.cgi?id=754687