Avoid failure to start during login

[andrew-sayers]

If started during login, this service exits with a warning "cannot open display: :1". Systemd then restarts it successfully a few moments later.

Delay start until it is likely to succeed.

[andrew-sayers]

Note after further testing - basic.target is not stopped if you log out then immediately back in again, so this fix doesn't solve the problem for the most obvious test scenario. You should be able to confirm this fix by logging out, waiting about 30 seconds for your instance of systemd --user to time out, then logging back in.

This loophole could probably be fixed by adding e.g. After=dbus.service, but that feels like it would add real-world fragility for the sake of hypothetical robustness.