GTK lost it's '+' suffix back in 2019, according to
<https://mail.gnome.org/archives/gtk-devel-list/2019-February/msg00000.html>

This commit can be re-generated with:

    git grep -l GTK+ \
    | grep -v -e ^NEWS -e ^glib/tests/collate.c \
    | xargs sed -i 's/GTK+/GTK/g'

Most of the changes are in comments and documentation.