GtkWidget: Use gdk_window_get_children_for_user_data
GtkWidget: Use gdk_window_get_children_for_user_data

This makes iterating over the children a lot faster, as we're
not doing lots of intra-library calls and type checks. We're still
in some sence O(n^2) since we iterate over each child window for each
widget, but the profiles look much better.

alexlarsson/gtk,ahodesuka/gtk,jadahl/gtk,jadahl/gtk,jadahl/gtk,grubersjoe/adwaita,ebassi/gtk,bratsche/gtk-,jadahl/gtk,alexlarsson/gtk,davidgumberg/gtk,jessevdk/gtk,alexlarsson/gtk,Sidnioulz/SandboxGtk,Adamovskiy/gtk,Sidnioulz/SandboxGtk,ahodesuka/gtk,Lyude/gtk-,Lyude/gtk-,jadahl/gtk,ahodesuka/gtk,jadahl/gtk,chergert/gtk,Lyude/gtk-,bratsche/gtk-,grubersjoe/adwaita,chergert/gtk,chergert/gtk,msteinert/gtk,Distrotech/gtk2,jessevdk/gtk,Adamovskiy/gtk,davidgumberg/gtk,Distrotech/gtk2,Distrotech/gtk2,grubersjoe/adwaita,bratsche/gtk-,davidgumberg/gtk,jessevdk/gtk,ebassi/gtk,Lyude/gtk-,alexlarsson/gtk,jessevdk/gtk,grubersjoe/adwaita,chergert/gtk,bratsche/gtk-,Adamovskiy/gtk,jigpu/gtk,Sidnioulz/SandboxGtk,Lyude/gtk-,Distrotech/gtk2,jigpu/gtk,jigpu/gtk,jigpu/gtk,Adamovskiy/gtk,bratsche/gtk-,bratsche/gtk-,Adamovskiy/gtk,davidgumberg/gtk,chergert/gtk,jessevdk/gtk,grubersjoe/adwaita,ahodesuka/gtk,jigpu/gtk,Adamovskiy/gtk,Sidnioulz/SandboxGtk,alexlarsson/gtk,grubersjoe/adwaita,msteinert/gtk,davidgumberg/gtk,Lyude/gtk-,jadahl/gtk,msteinert/gtk,alexlarsson/gtk,Lyude/gtk-,ahodesuka/gtk,chergert/gtk,ebassi/gtk,Adamovskiy/gtk,ebassi/gtk,Distrotech/gtk2,Distrotech/gtk2,msteinert/gtk,Sidnioulz/SandboxGtk,grubersjoe/adwaita,Lyude/gtk-,davidgumberg/gtk,ahodesuka/gtk,jigpu/gtk,Sidnioulz/SandboxGtk,alexlarsson/gtk,ebassi/gtk,chergert/gtk,davidgumberg/gtk,jadahl/gtk,Adamovskiy/gtk,msteinert/gtk,davidgumberg/gtk,chergert/gtk,jigpu/gtk,ahodesuka/gtk,msteinert/gtk,jessevdk/gtk,jessevdk/gtk,alexlarsson/gtk,grubersjoe/adwaita,jigpu/gtk,ahodesuka/gtk,ebassi/gtk