This implements QX11Info again, for source compat with Qt4, on top of
the XCB QPA backend.

The API and the documentation are forward-ported from Qt4 (minus things that
cannot be implemented anymore).
The implementation, however, is new (by Richard Moore, in this initial commit).

Change-Id: Iff9fd16fa6aaaa54f6d3cc0843b6b6fd08d5b1fb
Reviewed-by: Richard J. Moore <rich@kde.org>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>