waybar: integrate with tray.target (#6675)

Inspired by [#3432][0], but with a smaller footprint.

This change makes it so that services with `Requires=tray.target`
and `After=tray.target`, such as [`blueman-applet`][1] or
[`network-manager-applet`][2] get started *after* waybar is ready.

[0]: https://github.com/nix-community/home-manager/pull/3432
[1]: 6c2b79403e/modules/services/blueman-applet.nix
[2]: 6c2b79403e/modules/services/network-manager-applet.nix

tests/modules/programs/waybar/systemd-with-graphical-session-target.service

@@ -1,4 +1,5 @@
 [Install]
+WantedBy=tray.target
 WantedBy=sway-session.target
 
 [Service]
@@ -13,3 +14,4 @@ ConditionEnvironment=WAYLAND_DISPLAY
 Description=Highly customizable Wayland bar for Sway and Wlroots based compositors.
 Documentation=https://github.com/Alexays/Waybar/wiki
 PartOf=sway-session.target
+PartOf=tray.target