Add some missing properties to Deleted

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.

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