GtkExpander Constructor
GtkExpander::new_with_mnemonic (string label);
This is a static method which creates a new GtkExpander instance using label as the text of the label. If characters in label are preceded by an underscore, they are underlined. If you need a literal underscore character in a label, use '__' (two underscores). The first underlined character represents a keyboard accelerator called a mnemonic. Pressing Alt with that key activates the button. If label is Null the expander will have no label.
This method is a shortcut, you can achieve the same results by creating a new GtkExpander instance with a label containing an underscore and then calling the set_use_underline()
This method must be called statically.