Some configuration for Paperbox

I’ve added a simple Glib::KeyFile-based configuration “system” to Paperbox. It’s invisible when you’re using it, I just wanted it to store the application window layout, so you don’t have to do it every time you run the program.

I once pondered whether the program will ever have a settings dialog, and would details such as a color picker for tags would be useful. Well, not for now. I’ll try to keep it simple and effective and only add things which I feel are necessary for a better browsing and organizing experience.