Language codes in ISO 639-1 consist of two letters, those in ISO 639-3
of three letters. Use the appropriate domain for translations.

Fixes: #197

GnomeBGSlideShow does not need GDK, it only needs GIO.

GTimeVal is not year 2038 safe, and it has been deprecated in GLib.

We only need the warnings.

Version 0.49 is ancient.

Debian stable ships with 0.56.2, so let's go with that.

The get_pkgconfig_variable() method on the Dependency object was
replaced by a generic get_variable() method.

GTimeVal is not Y2038-safe, and has been deprecated in GLib.

When building against GLib ≥ 2.70, use g_spawn_check_wait_status(); when
building against older versions of GLib, use a compatibility shim.

The IdleMonitor class does not need GDK at all.

Something that doesn't look like it was written 3 years ago.

The test will have to be rewritten for GTK4 anyway.

For GNOME components using GTK 4, we provide a base library that does
not depend on GTK, libgnome-desktop-4, with the bits that do not use GDK
or GTK.

This library has a new soname and version, as well as a new pkg-config
file and introspection data files.

We want to keep libgnome-desktop-3 as a legacy library for GNOME
components (and some applications) that still use GTK 3; for this
reason, libgnome-desktop-3 will not change.

So we'll be able to add a dependency object on GTK4 later on.

The introspection scanner is perfectly capable of using the C type,
instead of the fully qualified type name.

Now that we split the non-GUI bits of libgnome-desktop into their own
library, we can provide a library for GUI ones.

For the time being, this is all still using GTK 3.

We don't use the root GdkWindow anywhere.

We don't really need a "GUI" library, especially since neither GnomeBG
nor GnomeRR really deal with user interfaces.

The end goal is to move GnomeBG and GnomeRR out of libgnome-desktop,
given that they are system component dependencies. For the time being,
though, we can safely keep them in tree, with each part of their own
shared library.

In order to port the GnomeBG and GnomeRR APIs to GTK4 we need to copy
the files into their own subdirectories, as we want to keep the older
GTK3-based implementations available for the legacy
libgnome-desktop-3.0. It also makes it easier for us to spin off these
libraries into their own projects, if we decide to do so.

We want to allow building libgnome-desktop without the GUI libraries.

At some point, we're going to build only the 4.0 libraries except on
older versions of GNOME.

- Do not include "config.h"
- Leave conditionally including xlocale.h to source files
- Add an error for missing GETTEXT_PACKAGE
- Rename non-idiomatic `_l` macro to `L_`

Otherwise xgettext won't recognise the translatable language string.

Fixes: #202

The `Makevars` file is only meant to be used by `make`.

If the project is configured to build without systemd support, this
function is unused.

If one is foolish enough to also build with -Werror, as I am, then this
the resulting unused-function warning is fatal and one is sad.

Previously, we used fedora:rawhide for gnome-desktop CI, but stopped
because gitlab.gnome.org's docker version was incompatible with F35
containers for a long time, so we pinned to F34 instead. Now we have
fixed that and updated to F35, but we'll be stuck here for a while
because we won't remember to update again in the future. fedora:latest
seems like a nice compromise between pinning to a particular stable
release vs. using rawhide for CI.