Add key mapping for the power button

On Ubuntu (I haven't tested other varients of Linux) pressing the power button while XBMC is fullscreen sends a <power> keypress. At the moment keyboard.xml has no mapping for this keypress, so this commit adds a mapping to XBMC.Shutdown.

NB in XBMCBuntu the power button already shuts down the system. It's possible this mapping could interfere with this, but I have no way of testing this.

See http://trac.xbmc.org/ticket/10004 for the original problem report.