Fix case of "OverlayUnwatched.png" directory

The `ListItem.Overlay` will return the value `OverlayUnwatched.png` for an unwatched item, NOT `OverlayUnWatched.png`. On a case sensitive file system the unwatched overlays will not be shown.

 * [Source reference](https://github.com/xbmc/xbmc/blob/master/xbmc/guilib/GUIListItem.cpp#L207)
 * [Wiki fix request](http://forum.xbmc.org/showthread.php?tid=165530)