Added basic subfolder functionality

Added recursive function that goes through subfolders within the base folder, and loads and hides images found.

Added the file-plugin folder icon as interface element to show/hide these images.

Hardly tested (only OS X & Chrome), but should work fine.

TODO:
Needs a good look at interface and how to deal with the subfolders, currently adding them as sub-lists.

Signed-off-by: Eelke Feenstra <dev@eelke.net>