Can we unify `Gtk::Container#add`, `Gtk::Stack#add_named` and `Gtk::Stack#add_titled`?

Normally, it is better Ruby API that users don't need to use different methods for similar operations:

```ruby
# The API in this pull request
stack.add(widget)
stack.set_add_named(widget, "name")
stack.set_add_titled(widget, "name", "title)

# A better API
stack.add(widget)                 # gtk_container_add()
stack.add(widget, "name")         # gtk_stack_add_named()
stack.add(widget, "name", "title) # gtk_stack_add_titled()

# Another better API
stack.add(widget)                   # gtk_container_add()
stack.add(widget, :name => "name")  # gtk_stack_add_named()
stack.add(widget, :name  => "name",
                  :title => "title) # gtk_stack_add_titled()
```

It is better APIs that the Ruby script will work well if a programmer writes a script as he wants to write.