main: signal readiness by notifying systemd

This is useful if we want to be able to start some services depending
on i3, notably some script using the socket or third-party bars like
polybar. To make use of this change, user is expected to use the
following stanza:

```
[Unit]
Description=i3 window manager
PartOf=graphical-session.target

[Service]
Type=notify
ExecStart=/usr/bin/i3
ExecStopPost=/bin/systemctl --user stop graphical-session.target
Restart=on-failure
```

Something similar is already possible using socket activation. For
example, we could use:

```
[Unit]
Description=i3 window manager socket
PartOf=graphical-session.target

[Socket]
ListenStream=%t/i3.sock
ExecStartPost=/bin/systemctl --user set-environment I3SOCK=%t/i3.sock
```

And other units could `Requires=i3.socket`. Unfortunately, not
everything is using I3SOCK. Notably, `i3 --get-socketpath` does not
and that's what i3ipcpp is doing. An alternative would be to patch `i3
--get-socketpath` to use I3SOCK if present, however, this may be a bit
risky. Should we check the environment variable first or the root
attribute?

Another alternative not requiring any modification is to have a
dedicated `i3-session.target`:

```
[Unit]
Description=i3 session
BindsTo=graphical-session.target
Wants=wallpaper.service
Wants=wallpaper.timer
Wants=polybar.service
Wants=i3-companion.service
```

And trigger it from i3:

```
exec_always --no-startup-id systemctl --user start --no-block i3-session.target
```

The proposed change being quite small, it seems harmless and
low-maintenance.

Signed-off-by: Vincent Bernat <vincent@bernat.ch>

RELEASE-NOTES-next

@@ -45,6 +45,7 @@ option is enabled and only then sets a screenshot as background.
   • Add 'all' window matching criterion
   • Acquire the WM_Sn selection when starting as required by ICCCM
   • Add --replace command line argument to replace an existing WM
+  • Notify systemd when i3 is ready, allowing other services to use i3 as a dependency
 
  ┌────────────────────────────┐
  │ Bugfixes                   │