Autostart using systemd instead of gnome-session

[ryonakano]

Fixes #105

Use the new name meson.options instead of meson_options.txt that was used before meson 1.1: https://mesonbuild.com/Build-options.html

Checklists

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

• pkexec ls shows Polkit Agent Dialog
  • FYI it results like this instead of the dialog if the daemon is not running:

    user@elementary:~$ pkexec ls -la
    ==== AUTHENTICATING FOR org.freedesktop.policykit.exec ====
    Authentication is needed to run `/usr/bin/ls -la' as the super user
    Authenticating as: user,,, (user)
    Password:
    polkit-agent-helper-1: error response to PolicyKit daemon: GDBus.Error:org.freedesktop.PolicyKit1.Error.Failed: No session for cookie
    ==== AUTHENTICATION FAILED ====
    Error executing command as another user: Not authorized
    
    This incident has been reported.
    [ble: exit 127]
    user@elementary:~$