libkworkspace: disable logout/shutdown/reboot when ksmserver is not running
Without ksmserver, the logout greeter screen can't be opened, so monitor the service running status and consider it when judging the ability to logout/shutdown/reboot.
Test:
- killall -9 ksmserver
- Open Kickoff and check the logout/shutdown/reboot button is gone
- systemctl --user restart plasma-ksmserver
- Open Kickoff and check the 3 buttons are back