unstable: add primary-selection protocol

This primary selection is similar in spirit to the eponimous
in X11, allowing a quick "select text + middle click" shortcut
to copying and pasting.

It's otherwise very similar to its Wayland counterpart, and
explicitly made consistent with it.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Carlos Garnacho <carlosg@gnome.org>