gui: Change View|Data and View|Variables to radio button menu items.

gui: Change View|Data and View|Variables to radio button menu items.

I didn't know what View|Variables did until I tried it, at which
point I was surprised that it changed into View|Data.  I think that
a pair of radio button menu items better communicates the intent.
Also, this situation seems to fit well what the GNOME Human Interface
Guidelines say about radio button menu items:

    Radio button menu items show which of two or more
    mutually-exclusive settings are currently selected, and allow
    the user to choose a different setting by selecting its menu
    item.

    * If you need to offer a choice of two mutually-exclusive
      settings to the user, use a group of two radio button items
      instead of a single check box menu item if the settings are
      not clearly opposites. For example, represent View as Icons
      and View as List as two radio button items.