[python/json/appmessenger] Announce DPMS state change to XBMCMonitor and JSON

Follow-up to https://github.com/xbmc/xbmc/pull/4760, enables addon devs to add onDPMSActivated() and onDPMSDeactivated() methods to their XBMCMonitor classes or just take the JSON notification if they like to be informed if the DPMS state changes (messages are sent from CApplication::ToggleDPMS() ).