The test deletes instances of QMimeData in the dropMimeData function.
The compiler warns about deleting objects of incomplete type if QMimeDate
is only forward declared.
Change-Id: I3423a7ea334180ff0b68efbecb3d3feeb0632239
Reviewed-by: David Skoland <david.skoland@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>