In View mode, dragging can only ever pan a zoomed in image. In all other
cases it does nothing. This patch initiates a drag drop operation
instead in those other cases, but keeps the panning functionality.
This works for videos as well as images, and also functions in
comparison mode.
When dragging a document, we check if it is zoomable, and if so whether
any part of it is not visible. If both are true, we assume panning is
possible and therefore abort the drag drop operation.
I wanted to display a drag pixmap of the image, but not exclude videos,
so I added a sort of generic `thumbnailRepresentation` function, which is
implemented differently for each adapter type. Videos simply return an
icon converted to a `QPixmap`.
BUG: 386034
FIXED-IN: 18.08.0
{F5781358}