Mnemonics implemented fully for menus; will appear on standard buttons

Mnemonics implemented fully for menus; will appear on standard buttons
and the RootPane menubar, but they will not be active, as they are
actually not mnemonics. They are a form of implicit ALT-X accelerators
(which use InputMap). Accelerators are next to implement. Should be
relatively easy.  Not planning to implement mouse-down-and-hold for
mnemonic cycles (multiple mnemonics with the same letter).