Use accessor functions instead direct access.

GTK+ 2.17.10 is now the required version
I've used all the GTK+ 2.18.* api available, still missing:
GTK_WIDGET_UNSET_FLAGS (widget, GTK_TOPLEVEL);
GTK_WIDGET_SET_FLAGS (widget, GTK_REALIZED);
GTK_WIDGET_REALIZED ()
GTK_WIDGET_MAPPED ()
GTK_VIEWPORT ()->bin_window
GTK_ENTRY ()->editing_canceled

https://bugzilla.gnome.org/show_bug.cgi?id=594957