Fix accessing data files

The standard path for all data files is data/ when using a package.js
layout. Also, we need to load GtkBuilder definitions from resources,
not files.
Fix traslations

We need to configure in the po/ directory and add a POTFILES.in file.
Also, we need to initialize Gettext to be able to use _()
Fix loading the CSS stylesheet

Include it in the resource, and load it at application startup.
Add missing Makfile