Add devel/gtk-layer-shell

A library to write GTK applications that use Layer Shell.
Layer Shell is a Wayland protocol for desktop shell
components, such as panels, notifications and wallpapers.
You can use it to anchor your windows to a corner or edge
of the output, or stretch them across the entire output.
It supports all Layer Shell features including popups and
popovers. This Library is compatible with C, C++ and any
language that supports GObject introspection files
(Python, Vala, etc, see using the library below).