Guake tabs start user sessions which don't end when closing tabs.

Each time I open a new tab in Guake, the number of users reported by the `users` command increases. Closing tabs does not decrease the number. Example, for user named `foo` after opening two guake tabs:

```sh
users
foo foo foo
```
^ that's one session after logging in, and one session for each of the two guake tabs.

Workaround: `sudo killall guake`

This is unlike the behavior of the built-in terminal, where the output of `users` is unaffected by the number of tabs.

```sh
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 20.04.1 LTS
Release: 20.04
Codename: focal

$ gnome-shell --version
GNOME Shell 3.36.4

$ guake --version
Guake Terminal: 3.6.3
VTE: 0.60.3
VTE runtime: 0.60.3
Gtk: 3.24.20

```