We already use the rename argument in install_data, so we should bump the
dependency on meson 0.49, however given that most of GNOME already
depends on 0.53 and given that it provides various nice features to be
used soon, it's just better to depend on such new version

Don't leave to humans what machines can do, the gnome.session.desktop.in.in
is filled with an hard-to-maintain single-line semicolon separated list of
components, that then are repeated in the gnome systemd gnome.session.conf.
And these are meant to be kept in sync.

Meson can help us in this, so let's just keep a single list in meson and
use it to generate the various formats we need.

Using a map to keep the components so that this can be easily adapted and
possibly new sessions can be defined

As per previous commit we fill the gnome-session's session.conf based on
a list of required components, we can also avoid hardcoding things even
more, and generate config files for all the configured desktop sessions
with required components.