st-widget: Add a proper paint, add st_widget_paint_background

Since we want to paint children by default in StWidget, we need to
provide a way for custom subclasses to paint their CSS backgrounds
without painting children... introducing st_widget_paint_background.

Additionally, remove any custom paint/pick handlers added by subclasses
of StWidget that just painted their children. This will cause double
painting if left alone.

This also removes the hacky things that some subclasses of StBin did
to prevent their one child to be painted by StBin.

https://bugzilla.gnome.org/show_bug.cgi?id=670034