Corrected strings in externs.m

[Fred Kiefer]

While chasing after a bug that Riccardo reported (The fix was intended
as a birthday present, but now I am too late), I noticed that many of
the strings used in the GUI file externs.m are different from their
Cocoa counterparts. This in itself is no big deal as we normally refer
to them via their names, which are the same. But when exchanging a NIB
file between these two systems the actual values get used.
The wrong values come mostly from a time when no GNUstep developer had
access to a Next or later Apple machine to test the actual value of
constants.

To prevent future problem I went through most of the file and wrote some
simple code to report the actual values on Cocoa. I did this for about
140 strings from this file and found plenty of differences, which I
corrected. The big problem now is that when you switch to the new GUI
code you will not only need to recompile all your GNUstep applications
and tools, in addition you may also need to regenerate all Gorm files
that include any of these strings. I am not sure whether there are any
at all, but potentially there could be some.

As usual with bulk changes like this, I might have overlooked a few
cases and/or make mistakes in the changed I did. Please feel free to
check and improve them.

Sorry for the inconvenience,
Fred