Reinstate layoutGetDrawWindow on Gtk+ 3.

Previously this was not supported on Gtk+ 3, but you need it if you want to do any custom drawing in a `GtkLayout` (which my application does). I think it's just an oversight.

`gtk_layout_get_bin_window()` has existed since Gtk+ 2.14 so I feel pretty
comfortable using it on both the 2 and 3 builds (and deleting code), but I haven't tested the Gtk+ 2 build…