x11/slick-greeter: Fix dependencies and prefix issues

[woodsb02]

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

Test Plan

poudriere testport = ok
Runtime test = ok, hidpi detection fixed

[ericbsd]

Thanks! Look good to me.