data: Leave to meson the duty of keeping the lists in sync
Don't leave to humans what machines can do, the gnome-initial-setup.session list is filled with an hard-to-maintain single-line semicolon separated list of components, that then are almost repeated in the gnome systemd 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.
A similar approach has been already done in gdm (gdm@cbd15652) and gnome-session (gnome-session@dd2ffb3d).
I didn't bump meson version here, but if we do we might simplify a bit the changes removing a few of join_paths and initializing the configuration data using a dict.
So, let me know if you want to do that as well.