Ok, there we are: I installed lxde-core without suggests and recommends. lxsession brings a graphical logout via lxsession-logout. This only works with libpolicykit-1 installed which in turn depends on libpam-systemd and therefore sytemd. edit: It's the same with libpolkit-backend-1-0 and libsystemd0. I reduced the systemd packages to recommends for both and policykit-1 installed fine. The logout GUI still didn't come up but segfaulted instead. strace revealed that I had to install lsb-release. The GUI came up but without buttons for shutdown or reboot. I will investigate that later.