[workspaces] Don't use a clone for the window drag actor

[workspaces] Don't use a clone for the window drag actor

Using a clone for the drag actor causes the animation into
the final position not to work as expected; instead use
the newly added DND parameters to change the original
drag actors size and opacity. (This reverts the change in
011db9f).

Since we are using the original actor, we have to be careful
not to change it during the drag, so don't actually position
dragged window actors in Workspace.positionWindows().

https://bugzilla.gnome.org/show_bug.cgi?id=613367