gtk_widget_set_visual and friends are not bound

[Daniel Silverstone]

At least gtk_widget_set_visual() is not bound, but I imagine other visual related functions may be missing.

[Guillaume Gomez]

set_visual is bound: https://github.com/gtk-rs/gtk/blob/master/src/auto/widget.rs#L2686

I'm closing this issue so don't hesitate to reopen for the other missing functions.