Fix moving and undo-moving a selection of widgets.

The current status of this issue in Umbrello 2.11.0 is almost right: it works as expected but undo works only for one of the selected items.
On master though, there seems to be some regression: moving 5 items 200 pixels will cause only 1 item to move to move - by 5*200 pixels!

CCBUG: 291747