Another round of template binding api changes

Another round of template binding api changes

We rename the gtk_widget_class_bind_template_child{_internal}
macros by appending a _private to their name. Otherwise, it
would be too magic to pass the 'public' names as arguments,
but affect a member of the Private struct. At the same time,

Add two new macros with the old names,
gtk_widget_class_bind_template_child{_internal} that operate
on members of the instance struct.