Replace --autostart with autostart whitelist

So, currently we have this weird --autostart option, which is a PITA if we want to use a generator for autostart.

So, this feature exists to autostart a few applications even if autostart should be disabled in general. Really, this is only relevant for the login screen. People have tried to e.g. use --failsafe in the past, but disabling all autostart affects e.g. orca, which is not really desired either. One could argue that it should be a RequiredComponent though.

Either way, effectively we have one use case for this feature. That is starting orca-autostart and spice-vdagent in the greeter. Both install desktop files to /etc/xdg/autostart already, and really, all we want here is whitelisting them.

So, doing it this way is simpler (i.e. 6 files changed, 27 insertions(+), 68 deletions(-)). And, the great thing is, that we can read AutostartWhitelist in the generator, allowing us to modify the systemd session definition to explicitly pull in only those components rather than xdg-desktop-autostart.target which would start everything.

Can we do this? It is an incompatible change technically. It may burden distributions a bit (need to patch gnome-session.login unless we just add spice-vdagent upstream). But it does simplify things and removes some of the magic between GDM and gnome-session.

See gdm!94 (closed) for the follow up change there (neither of these changes is actually tested currently).

CC: @iainl