Rename GTK+ to GTK (mostly comments and documentation)

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.

docs/reference/glib/building.xml

@@ -49,7 +49,7 @@
       aliasing and cannot be guaranteed to work.
     </para>
     <para>
-      The GTK+ documentation contains
+      The GTK documentation contains
       <ulink url="https://developer.gnome.org/gtk3/stable/gtk-building.html">further details</ulink>
       about the build process and ways to influence it.
     </para>