If set, an underline in the label property indicates that the next character
should be used for the mnemonic accelerator key in the overflow menu. For
example, if the label property is "_Open" and use_underline is TRUE,
the label on the tool button will be "Open" and the item on the overflow
menu will have an underlined 'O'.
Labels shown on tool buttons never have mnemonics on them; this property
only affects the menu item on the overflow menu.
Since 2.4
If set, an underline in the label property indicates that the next character should be used for the mnemonic accelerator key in the overflow menu. For example, if the label property is "_Open" and use_underline is TRUE, the label on the tool button will be "Open" and the item on the overflow menu will have an underlined 'O'. Labels shown on tool buttons never have mnemonics on them; this property only affects the menu item on the overflow menu. Since 2.4