[powermanagement] Rework kcm, add dpms support, fix "Never" option

Delete the config groups if we select the "never" option. Add DPMS support, because the only two options that were being configured before this was suspend and screen dimming, NOT when the screen would turn off (which is always set to 60 seconds).

This also fixes the power management settings only applying on battery mode, and not in low battery or AC/plugged in mode.

Potential fix for #11 (closed)