Doing this allows us to automatically load stylesheets located in the resource base path, so move contents of both Adwaita.css and nautilus.css to style.css in the root of the resource directory, which will be always automatically loaded.
Additionally, create a style-hc.css which will be used when the system high contrast preference is enabled, and adjust it to fix issues with NautilusPathBar.
Fixes #2166 (closed)