Adds QWindowsDropDataObject as a QWindowsOleDataObject subclass
specialized for handling Drag & Drop, which only allows "text/uri-list"
data to be exported as CF_HDROP, to allow files dragged from
Qt applications and dropped into other applications to be recognized as
attachments or opened. Otherwise, files would be imported as URLs and
inserted as local hyperlinks.
[ChangeLog][Important Behavior Changes][Windows] On Windows, a drag
& drop operation of local files URIs, like QListView items backed by a
QFileSystemModel, will result in the attachment or opening of the files
by the target application, instead of the creation of hyperlinks.
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>