Bug 737791 - Adapt to changed GtkStateFlags semantics for expanders

GTK 3.13.7 broke backward-compat on which state flag controls how an
expander is drawn.

Older versions used GTK_STATE_FLAG_ACTIVE, 3.13.7 and later changed it
to GTK_STATE_FLAG_CHECKED.

See https://bugzilla.gnome.org/733967 for details.