use_underline : TRUE if underscores in the text indicate mnemonics
The set_use_underline() method sets the "use_underline" property to the value of use_underline. If use_underline is TRUE, an underscore in the text of the button label indicates that the next character should be underlined and used for the mnemonic accelerator key if it is also the first underlined character.
See also: get_use_underline()