Improve Human-Computer Interation of PVR EPG

Furter to the feature request:
http://forum.xbmc.org/showthread.php?tid=154675&pid=1383179#pid1383179
This commit aims to address the second change request therein.  Pressing
the "OK" button over an EPG entry that is currently playing (i.e. Live),
the default behaviour switches straight to that channel. However, EPG
entries that are not currently playing (i.e. future timeline entries)
then the behaviour, "Programme Info" pop-up context menu appears to
give the user the option to set a timer for this future event.
Clear the EPG when switching between channels

If a channel is being watched live and a user calls the EPG, on switching to
a new channel the EPG remains on the screen.  This commit clears the EPG on
channel change.