add additional power management settings if docked vs just plugged into external power or on battery

Although we have #202 (closed) , xfce4-power-manager tries to inhibit logind and mimic HandleLidSwitch and HandleLidSwitchExternalPower that is configurable in logind.conf. logind.conf also has an option for HandleLidSwitchDocked which "If the system is inserted in a docking station, or if more than one display is connected, the action specified by HandleLidSwitchDocked= occurs."

I'd like to have this same option in xfce. If you plug your laptop into a docking station and close the lid, there are many times where you don't want it to go to sleep. However, normally if you plug your laptop into just a charger, you do want it to go to sleep when you close the lid.