Rework move operation to use offsets

[amolenaar]

On macOS, objects sometimes jump back to their starting position. This is because the event sequence is closed before the drag-end handler is fully executed. It's an intermittent error.

However, for simplicity and cleanness of interface it would be nice to provide the move operations (move and stop_move) with offsets, instead of actual positions.

See #891 for the current solution: remember the start position ourselves.