Support QImage and others in Wayland clipboard.

Support QImage and others in Wayland clipboard.

The wayland plugin's clipboard implementation lacked support for data
set via QMimeData::setImageData, QClipboard::setPixmap, etc. because
everything was treated as a QByteArray. A variant having an image
cannot however be converted to a byte array. Therefore, to make these
convenience functions to work, the image is now encoded via
QImageWriter before sending it. On the receiving side QMimeData is
capable of decoding images from byte arrays, so no problem there.
The patch also renames an internal QWaylandMimeData class to a more
suitable clipboard-specific name as the functionality is clipboard
specific.

Change-Id: I77e7df903b8dbe9731613fdbb9693b2a37b05d18
Reviewed-on: http://codereview.qt.nokia.com/2616
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Jørgen Lind <jorgen.lind@nokia.com>