Summary:
### keepAbove

Some effects(e.g. Dim Inactive) can take keep above state of a window
into account when they are making decision whether to operate on it.
Because Deleted doesn't expose keepAbove property, it will be always
`true`, which is wrong.

### keepBelow

This property was added as a counterpart to keepAbove.

### caption

That's mostly for debugging purposes, e.g.

```lang=cpp
void CoolEffect::windowClosed(EffectWindow *w)
{
    qDebug() << w->caption() << "has been closed";
    qDebug() << "keep above:" << w->keepAbove();
}
```

Test Plan: Manually.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13649