Improve feedback for moving executions

The feed-back presented when moving executions has several limitations:

- when dragging an execution to another lifeline, the feedback rectangle does not move until the pointer hovers over the destination lifeline. It should track the pointer _en route_
- when dragging an execution to another lifeline, the feedback rectangle is locked to the original vertical (y) position of the execution. Because changing lifelines requires the `Cmd` modifier key (`Ctrl` on Winux), it can be positioned anywhere on the destination lifeline, so the feedback should track that
- when dragging an execution onto a stack of existing executions (pending pull request #423), the feedback rectangle is shown on the lifeline stem. It should instead be shown on the right edge of the execution that would be its parent if it will not become the root execution