While developing our editor control we tried to implement DnD of
Text-Selection to:
* to rearrange text inside the control
* to copy text selections to another position inside the text

On Win32 everything works as expected, if you start dragging the initial
operation is MOVE and you can change that to a COPY by pressing CTRL
while dragging.

On Linux the initial gesture is COPY which is not how DnD works in
things like nautilus, ... . I can get that to change to a move operation
by holding the SHIFT key.

On OS-X the initial gesture is a COPY and which is not how DnD works
finder or eg Finder, and I haven't found a way to change the operation
to a MOVE.

I would expect the following on ALL operation systems:
- a drag WITHOUT any modifier key is a MOVE operation

- on Windows & Linux I make it to a copy operation by pressing CTRL
  while dragging

- on OS-X I make it a copy operation by pressing ALT while dragging

In general I think the problem is even bigger because the default
operation depends on the target eg on windows when you drag a file
between 2 folders on the same filesystem you get a default MOVE but if
you drag to another one the default is a COPY and SHIFT makes it a MOVE.