Use systemd to handle more of the shutdown

Previously we killed kwin manually, then tried to exit the session. This is racey as kwin will close all connections, before systemd has realised that the session is ending this can cause all apps to restart.

Also closing plasma-workspace-wayland.target is ineffective, closing the graphical-session implicitly brings down the session more explicitly. It'll close the other targets and services.

BUG: 473824 BUG: 409088 BUG: 450531 BUG: 451939 BUG: 450320 BUG: 434403

Tested by looking at coredumpctl list after a logout and seeing it go from 10 entries of silly aborts restarting without a display server to 3, with different unrelated causes.