Sets the menu item's label. If the item has a shortcut, the shortcut is not affected.
If the label contains an ampersand (``&'') and the window is a control, the label is parsed specially; under Windows, the character following an ampersand is underlined in the displayed menu to indicate a keyboard mnemonic. Pressing and releasing the Alt key switches to menu-selection mode in the menu bar where mnemonic characters are used for navigation. (An Alt combination might select a menu via on-menu-char.) A double-ampersand in the label is replaced by a literal (non-navigation) ampersand. Under X and MacOS, ampersands in the label are parsed in the same way as for Windows, but no mnemonic underline is displayed.
An ampersand is always preserved in the label returned by get-label, but never preserved in the label returned by get-plain-label.