Dynamically add menu mnemonics to all menus

Added and called `Toolkit.setMenuMnemonics(JMenuItem...)` and `Toolkit.setMenuMnemonics(JMenuBar)`. These automatically set menu mnemonics based on the KDE, Windows, and Gnome human-interface guidelines, except on Mac, where as per Mac HIGs, it does nothing.
Really, there should be a `Toolkit.setMenuMnemonics(JPopupMenu)`, but I couldn't make one, so I've called
`Toolkit.setMenuMnemonics(firstItem, secondItem, thirdItem, ...)` instead.
#51