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/gio/overview.xml

@@ -154,7 +154,7 @@
   </para>
 
   <figure id="gvfs-overview">
-    <title>GIO in the GTK+ library stack</title>
+    <title>GIO in the GTK library stack</title>
     <graphic fileref="gvfs-overview.png" format="PNG"></graphic>
   </figure>
 
@@ -166,7 +166,7 @@
   </para>
   <para>
     One of the big advantages of putting the VFS in the GLib layer
-    is that GTK+ can directly use it, e.g. in the filechooser.
+    is that GTK can directly use it, e.g. in the filechooser.
   </para>
   </chapter>