Add in capacity for a SortableListPreference

I've added a new demo which I think will be of some interest. I know I'm planning on using it in my current project a couple of times. It's a SortableListPreference, which I've added to the demo activity. I think it will be of use to others, so I'm handing it back to you with the hopes that you can add it to your project, making it available for others to use as well.