tooltip custom group: duplicate labels

[Nick Roberts]

 > The tag "Use echo area" is used twice in the same customization buffer.
 > 
 > Its first occurrence comes from emacs/lisp/tooltip.el:
   ...
 > and the second occurrence comes from emacs/lisp/progmodes/gud.el:
   ...
 > I propose to remove :tag from all GUD tooltip options to reveal their
 > true identities to the users of Customize.

I don't see the need to remove them both, so I've just removed the tag for
gud-tooltip-echo-area.  I've also added the gud groupname for the customize
variables for GUD tooltips and moved tooltip-mode and gud-tooltip-mode forward
so that they appear first in their part of the customize buffers.  I think the
first question to ask is "Do you want the mode on or off".  I don't think this
causes a problem as no byte compile errors occur.

Nick