[infobools] Expose m_dpmsIsActive to skins/plugins/scripts as "System.DPMSActive"

See title.

Background: I was asked if script.xbmc.lcdproc can do some sort of action (e.g. turn off display backlight) when DPMS energy saving stuff kicks in, this'll make it possible to provide such a feature. Might even come in handy for other service scripts or so.