Summary:
984b9737 enabled dragging from {nav View} mode, but the cursor
stayed the same as when panning. This is because by default
`QGraphicsView` sets the cursor for moving objects, i.e.
`ClosedHandCursor`. By setting a default action for `QDrag::exec`, we
get the `DragCopyCursor` which better indicates that the image can be
dropped, and which is also used when dragging from the `ThumbnailView`.

When drag-and-dropping within Dolphin, you can press {key Shift},
{key Ctrl} or {key Ctrl Shift} to directly move, copy or link a file
instead of showing the popup menu asking for the desired action. By
setting all three actions for the drag object, we can enabled this
behaviour for dragging from Gwenview to Dolphin too, instead of always
defaulting to copying.

Test Plan:
Press {key Shift Ctrl} while dragging an image from either {nav View} or
{nav Browse} mode to Dolphin. A cursor indicating the linking action
should be shown instead of the panning cursor.

Reviewers: #gwenview, huoni

Reviewed By: #gwenview, huoni

Subscribers: huoni

Differential Revision: https://phabricator.kde.org/D13725