Add new Built-In function to control attached CEC device

Hi,

this is my first contribution to xbmc. They're a couple patches targeted at USB CEC
or RaspberryPi owners:

 - A new Built-In method to control the state of the attached playing CEC device,
   thought to be mapped to a remote key.

 - Support for a new libcec method, PowerOnScreensaver, to control whether
   waking the CEC device when waking the screensaver.