bind 2D cursors if changed by display interactor
If the 2D cursors (aka crosshair) and there are multiple images open then the cursors should move in sync in each image.
This works if the the cursor position is changed by left mouse button (handled by the slice navigation controller state machine internally) but not if it's changed by middle or right mouse button (panning or zooming, handled by the display interactor).
This is a consequence of a recent redesign of the display interactor in midas. Earlier the crosshair was not moved at panning or zooming.