Summary:
Currently, there is a guarantee that a client, which is about to be removed,
is no longer in the stacking order(both in constrained and unconstrained)
when Workspace::removeClient is called. However, because the client gets
removed from m_allClients after clientRemoved is emitted, it can be
re-inserted back into the stacking order.

In general, the pattern is to do some work and then notify others about
what you've done by emitting a signal. In the case of Workspace::removeClient,
we emit clientRemoved way before the client actually gets removed.

CCBUG: 392412
CCBUG: 400854

Test Plan:
* Enable the following script:

```lang=js
workspace.clientAdded.connect(function (client) {
    if (client.skipTaskbar || client.modal || client.transient) {
        return;
    }
    workspace.desktops = workspace.desktops + 1;
    workspace.currentDesktop = workspace.desktops;
    client.desktop = workspace.currentDesktop;
});

workspace.clientRemoved.connect(function (client) {
    if (client.skipTaskbar || client.modal || client.transient) {
        return;
    }
    workspace.desktops = workspace.desktops - 1;
});
```

* Open an app, close the app.

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: graesslin, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17069