Autostart using systemd instead of gnome-session

[ryonakano]

Fixes #217

Backport of the fix in NixOS: NixOS/nixpkgs@d1de5b7

Checklists

NOTE: You'll need to run systemctl enable --user io.elementary.settings-daemon.service after building and installing this branch, while this should be automatically handled in case of Debian packaging.

• Wallpaper is synced between the desktop and Greeter after logging out or rebooting
• Color scheme preference is synced between the desktop and Greeter after logging out or rebooting
• Custom keyboard shortcut works (as reported in nixos/pantheon: Custom keyboard shortcut not working NixOS/nixpkgs#478319)

[ryonakano]

@bobby285271 I would appreciate it if you would review this and check if Settings Daemon autostarts on NixOS without NixOS/nixpkgs@d1de5b7.

[bobby285271]

After fixing these issues I can confirm this works for me:

1. The build fails on update-desktop-database because with this PR nothing is installed in share/applications any more.

   settings-daemon/meson.build

   Line 63 in 96a7f48

   update_desktop_database: true

   FAILED: install script '/nix/store/5ridz8xqki02m7zpjpnbnmx6pjgcc68d-desktop-file-utils-0.28/bin/update-desktop-database -q /nix/store/g56rp1b1484s1r1763isa8pwd6mhqddr-elementary-settings-daemon-8.4.0/share/applications' failed with exit code 1.
   

2. Then I got this in my journal that requires me to change the ExecStart line to an absolute path (i.e. ExecStart=@bindir@/io.elementary.settings-daemon)

   Jan 17 11:28:56 thinkbook (s-daemon)[4472]: io.elementary.settings-daemon.service: Failed at step EXEC spawning io.elementary.settings-daemon: No such file or directory
   Jan 17 11:28:56 thinkbook (s-daemon)[4472]: io.elementary.settings-daemon.service: Unable to locate executable 'io.elementary.settings-daemon': No such file or directory
   

(p.s. also hit a NixOS issue that is not related to this PR :-) We know how to workaround that though)

[ryonakano]

@bobby285271 Thank you for your quick test! I addressed your comments.