Add support for winaltkeys

- The winaltkeys=no setting disables the menubar mnemonics, and passes
  all Alt+mappings to Vim.
- Qt does no have a simple way to do this, so I've subclassed QMenu and
  added some state to switch the menu title, between the version with
  and without mnemonics.
- Currently this is only used for the menus in the menu bar, not the
  submenus - this seems to be the expected behaviour - the state change
  is triggered by a signal from the MainWindow object
* issue#84