x11/slick-greeter: Fix dependencies and prefix issues

Summary:

• Add dependencies harfbuzz, numlockx, python, dconf, pygobject3
• Fix slick-greeter-check-hidpi and slick-greeter-set-keyboard-layout commands by adding python dependency, fixing shebang paths, and installing under correct ${PREFIX} location
• Patch numerous other /usr and /etc prefix issues
• Add more detail to pkg-descr to support users choosing a greeter

Differential Revision: https://reviews.freebsd.org/D31631