Restructured themes to a separate folder to increase modularity for developers

Starting a series of pull request for some file structure changes for v4.0. Currently in version 3.1+, the many features including the themes are nested inside the src folder. This is ideal for many reasons, yet WET 4.0 is trying to segregate the core features, CSS, and functionality from theme-ing a bit more to simplify the creation of themes, and intuitive bundling for developers, potentially increasing implementers time to market.