Add a small systemd user target modelled on miracle-wm-session.target.
It binds to graphical-session.target and orders after
graphical-session-pre.target, so systemd user services declaring
WantedBy=graphical-session.target (panels, portals, notification
daemons, ...) start and stop in sync with a labwc session.
Installed into $systemduserunitdir when the systemd dependency is
available at configure time; on systems without systemd the install
is skipped and labwc's runtime activation of the target fails
gracefully.
