XBMC4Xbox

Feature requested on the forums by Byron to so can add "XBMC Subtitles" to a skin, see:
http://www.xbmc4xbox.org.uk/forum/viewtopic.php?f=15&t=2939&p=23273

System.HasAddon() boolean condition
http://wiki.xbmc.org/index.php?title=List_of_boolean_conditions

Please see attached a patch and a test script, please review.

Implemented was aimed at scripts and obviously we don't have an Addons database yet, so it checks for addon (script) directory on disk. It doesn't read and check the addon.xml in all addon folders (thought might be too expensive, especially if used with a skin), so it just deduces the location from the addon id, e.g. for "script.xbmc.subtitles" it checks "special://xbmc/scripts/xbmc subtitles".