Add isRunning property for epg grid

This adds 

<i>ListItem.IsRunning</i>

to the EPG Grid Container. It can be used to indicate if a program has already started, e.g. the left arrow in this <a href="http://f.cl.ly/items/1c200G2t0n172B2k2S1n/Screen-shot-2010-11-30-at-16.55.26.png">this</a> screenshot of the windows media center epg.