[bug#70508] [PATCH] services: Add gvfs-service-type.

[宋文武]

>From Patch1:
> * gnu/packages/xfce.scm (thunar)[build-system]: Switch to 
> glib-or-gtk-build-system.
>

>From Patch2:
> * gnu/services/desktop.scm (gvfs-service-type): New variable.
> (gvfs-configuration, make-gvfs-configuration, gvfs-configuration?,
> gvfs-package): New procedures.
> (<gvfs-configuration>): New record.
> * doc/guix.texi (Desktop Services): Document it.

Both patches are needed technically, as GIO applications needed to be
wrapped with GIO_EXTRA_MODULES; and udisks, gvfs available as dbus
services.  While it will still works with only Patch2, due to
gvfs-service-type also introduces GIO_EXTRA_MODULES via
gvfs-service-type -> udisks-service->type -> udisks -> glib dependencies
by chance, but on a foreign system or when the system side
libgvfsdbus.so is not ABI compatible with applicaiton's GLib, I guess
Patch1 could make application works as long as the GVfs dbus interface
is compatible.