GtkBin as a publicly derivable abstract class please.

[Tristan Van Berkom <tristanvb openismus com>]

Guys, 
   this is the kind of thing we could probably just
resolve on irc but I work an opposite schedule so I'm
taking it to the list.

Today I found '_gtk_bin_set_child()'

Making that a private api render's GtkBin completely
useless as a subclassable container outside of GTK+.

But I beg to differ, GtkBin is definitely a useful
abstract class to derive from.

Also I noticed that the implementation takes no
reference to the child.

Please lets not make the mistake of gtk_widget_set_window()
again, foo_class_set_bar() apis invariably take a ref
to the object 'bar'... accept for the awkward case of
gtk_widget_set_window().

My recommended course of action is that gtk_bin_set_child()
should not directly make the reference but instead do so
implicitly by way of gtk_widget_set_parent(), thus somewhat
further reducing the workload of GtkBin subclasses.

It should return_if_fail() if (bin->child != NULL) as well.

Regards,
      -Tristan