Desktop variables and caches

[Fabian Harfert]

Hi!

While working on and testing my MATE packages I noticed the following
things:

I already mentioned this before, but it belongs here, too: Gtk+ needs
the environment variable GTK_DATA_PREFIX defined to the user profile
directory to find themes.

For GdkPixbuf it's necessary to define GDK_PIXBUF_MODULEDIR to the
sub-directory lib/gdk-pixbuf-2.0/2.10.0/loaders of the users profile to
make it finding loader modules like the one provided by librsvg for
SVG support. In addition the command 'gdk-pixbuf-query-loaders' needs
to be called to cache the different loaders. For that the variable
GDK_PIXBUF_MODULE_FILE has to contain the cache file
(lib/gdk-pixbuf-2.0/2.10.0/loaders.cache). That must be done when the
profile is built.

To allow GLib GSettings to find it's compiled settings schemas the
command 'glib-compile-schemas' with the share/glib-2.0/schemas
directory needs to be called when the profile is built.

The last two points result in the fact, that each package using
extensions to GdkPixbuf or GSettings schemas from other packages needs
to propagate them (e.g. librsvg or gsettings-desktop-schemas).

We also need to define the XDG_CONFIG_DIRS variable. It is used by
several applications to find files in etc from other applications.

Please note that I'm still not using the Guix system distribution, so I
don't know if this also applies for that platform, but I think so.

Fabian