Fix for bug 14899

This is a fix for bug 14899 -  http://trac.xbmc.org/ticket/14899
When asking for Picture InfoLabels, the information is not available until it has been loaded from the exif and iptc tags in a file. When a list of pictures is built, the info is not loaded, so the $INFO[ListItem.Picture...] labels will be blank, until some other mechanism loads the info from the file. The tags should not be loaded when the list is built, as this could be a very big performance hit for large picture lists. However, once the list of pictures is displayed, I think the info should be available for the currently selected picture. This fix implements logic so that if Picture... Info is requested for a currently selected ListItem, and the ListItem is a picture, and the tags have not yet been loaded, then the tags will be loaded from the file before the info is retrieved.
Need for this fix is in http://forum.xbmc.org/showthread.php?tid=184976