Windows clipboard: Deny CF_DIB support for images with alpha.

Support CF_DIBV5 for these images so that transparency is preserved.

Task-number: QTBUG-11463
Change-Id: I51881ae8bfbd05b92abd309766f4da9a7ea26c2d
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>