* adds a sway-session.target that binds to graphical-session.target
(see man systemd.special)
* also adds a 10-systemd script that should (must?) be included from sway
that starts imports the sway environment variables into the systemd
--user instance, and then starts sway-session.target
The idea is that, any --user units that are wanted by either
sway-session.target or graphical-session.target will be started when
sway runs, and will have the correct environment from sway too.
