This patch adds drag functionality to View mode. A
drag operation is only initiated if dragging wouldn't
otherwise do something, e.g. when zoomed and the image is
pannable, or the crop tool is active.
In order to get a drag pixmap for all cases (videos
included), we use a ThumbnailProvider to generate a file
preview. However, if the image is modified, we generate a
new pixmap using the edited version. Using
MimeTypeUtils::selectionMimeData, this should mean the
drag pixmap always matches the image the receiving
application gets.
BUG: 386034
FIXED-IN: 18.08.0