Add gtk_widget_(un)register_window

This replaces the previously hardcoded calls to gdk_window_set_user_data,
and also lets us track which windows are a part of a widget. Old code
should continue working as is, but new features that require the
windows may not work perfectly.

We need this for the transparent widget support to work, as we need
to specially mark the windows of child widgets.

https://bugzilla.gnome.org/show_bug.cgi?id=687842

gtk/gtk.symbols

@@ -3839,6 +3839,7 @@ gtk_widget_queue_resize
 gtk_widget_queue_resize_no_redraw
 gtk_widget_realize
 gtk_widget_region_intersect
+gtk_widget_register_window
 gtk_widget_remove_accelerator
 gtk_widget_remove_mnemonic_label
 gtk_widget_render_icon
@@ -3909,6 +3910,7 @@ gtk_widget_trigger_tooltip_query
 gtk_widget_unmap
 gtk_widget_unparent
 gtk_widget_unrealize
+gtk_widget_unregister_window
 gtk_widget_unset_state_flags
 gtk_widget_insert_action_group
 #ifdef GDK_WINDOWING_WIN32