GTK+ has been renamed to GTK without the plus

[Vivien Kraus]

Dear gnus,

The coding standards refer to "GTK+" in 4 places, one being an index
entry. Nowadays the library is called simply "GTK" https://gtk.org/

Also it is referred to as "the GTK+ toolkit […]" which seems a bit
redundant, maybe: "the GTK library or the GNUstep toolkit"?

Finally, the standards discuss CORBA (it even has an index entry), is
that still relevant?

Best regards,

Vivien