install a systemd user unit

add a systemd --user unit/service file, so that one can run waybar as a
--user systemd service. when the service is enabled, Waybar will start
when the wayland-session.target is started.

this feature is automatically enabled if systemd is found, but can be disabled
with -Dsystemd=disabled

resources/waybar.service

@@ -0,0 +1,12 @@
+[Unit]
+Description=Highly customizable Wayland bar for Sway and Wlroots based compositors.
+Documentation=https://github.com/Alexays/Waybar/wiki/
+PartOf=wayland-session.target
+
+[Service]
+Type=dbus
+BusName=fr.arouillard.waybar
+ExecStart=/usr/bin/waybar
+
+[Install]
+WantedBy=wayland-session.target