x11-toolkits/gtk-session-lock: add new port

This is a library to use GTK 3 to build screen lockers using the
secure ext-session-lock-v1 protocol. This Library is compatible with
C, C++ and any language that supports GObject introspection files
(Python, Vala, etc, see using the library below).

This library is a fork of the incredible gtk-layer-shell, which has
laid all the groundwork necessary to make this happen.

https://github.com/Cu3PO42/gtk-session-lock