QtIFW

Revert change from QIODevice to QFileDevice.

Follow up on 924ebcdbc9b9e590a5f0905941e2d05ac34d4be2 (Part 2).
Remove some unused methods. Cleanup some unused code. Add some
documentation what the Archive class does and is used for. Add
copy method and switch back the class to inherit from QIODevice
as also the Archive class does not work as QFileDevice descent.

Change-Id: Ia20f02c5aa53ba977fdc1d14785031b0cfbce386
Reviewed-by: Kai Koehne <kai.koehne@digia.com>