add Gtk2::Layout->bin_window

["Philipp Rumpf" <prumpf gmail com>]

Gtk2::Layout has a publicly accessible (but read-only, I understand)
struct member called "bin_window", which is necessary to implement
expose events properly.

Unless I'm terribly confused, Gtk2-140 doesn't have an access function
for that, so I added one to GtkLayout.xs.  It appears to work.  Patch
attached.

Philipp Rumpf

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