GTK: Don't use gdk_window_set_user_data() in a deprecated manner
GTK: Don't use gdk_window_set_user_data() in a deprecated manner

Since GTK 3.8 gtk_widget_register_window() should be used instead of
gdk_window_set_user_data() in a widget's ::realize() method.

gtk_widget_unregister_window() is called by the GtkWidget's
::unrealize() handler we chain up to.

This has no functional impact.

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