Fixed bug in FileManager

When a non-image file was added to the directory being watched it would be added to the list. When a non-image file was removed from the directory being watched it would try to remove an entry with a negative index from the list. The same thing would happen with renaming. Those bugs are now fixed.
Started working on a WPF version

Although a good exercise, I'm not quite sure if I should really try to port this to WPF. PictureBox seems to be much better suited to my needs than Image (which can't play animated gifs) or MediaElement (which has flashes annoyingly when its source is updated).