Create a internal/read-only trayTarget option for the xsession, which is
also used in wayland's config, if the former is not enabled.
Remove all other definitions of `systemd.user.targets.tray`, i. e, the
ones from the following modules: hyprland, sway, river and wayfire.
The `exec` command does not do any shell parsing and does not
understand the `&&` which is how the extraCommands are added after
dbus activation. There doesn't seem to be a reason for `exec` here
anyway so just remove it and allow shell parsing.