* guix/build/glib-or-gtk-build-system.scm (data-directories): Rename
'schemas-directories' to 'data-directories' and add support for XDG theming
data.
* guix/build/glib-or-gtk-build-system.scm (gio-module-directories): New
function.
* guix/build/glib-or-gtk-build-system.scm (wrap-all-programs): Update names to
reflect that we are dealing with more types of data and not only with
schemas. Add handling of GIO modules.
* guix/build-system/glib-or-gtk.scm (lower): Import the 'bin' output of GLib
instead of 'out'. This was an error since we need the program
'glib-compile-schemas'. Update the description.