Turning display off while playing PVR-TV/PVR-radio

For the time being (no separate GUI-setting for 'DisableOnPlayback') this makes turning the backlight off more consequent within PVR-module-playbacks.

As described in
http://www.vdr-portal.de/board60-linux/board62-software/board95-xbmc/p1119078-wip-xbmc-lcdproc-support-als-python-addon-tester-gesucht/#post1119078
and
http://www.vdr-portal.de/board60-linux/board62-software/board95-xbmc/p1119171-wip-xbmc-lcdproc-support-als-python-addon-tester-gesucht/#post1119171

Cheers,
Chriss