BUG: Fix IconEngine and d&d of QStandardItems with icons

When doing drag&drop, the mime data failed to stream the QIcon because
qSlicerIconEnginePlugin was not a subclass of QIconEngineV2.
Regression when adding Qt5 support in CTK.
Closes #3778

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23487 3bd1e089-480b-0410-8dfb-8563597acbee