[systemd] Fix logout issues
It is important that we end plasma-core.target plasma-workspace.target and graphical-session.target when we log out. graphical-session in turn closes all other apps and daemons and prevents restarts. Handling it in systemd gives the least racey option. graphical-session used to be hanlded in the plasma-ksmserver service file, but this was removed in 5.24 in a refactor where it caused issues for Xwayland restarting. This patch reintroduces the sentinel programs to close down our target, which in turn take down the other targets. It also removes a complicated use of templates that in hindsight didn't really solve anything. BUG: 448492 Testing done: - logout of wayland shows all targets as inactive, logging in repeatedly works smoothly - logout of X11 behaves the same. - calling systemctl --user stop plasma-kwin_wayland.service closes the session - killing ksmserver on wayland does not close the session - killing ksmserver on X does