Issues with zooming/dragging and dropping elements on a Batch viewer diagram

[anxolin]

Related to gnosis/gp-ui/#1061

Open https://pr1061--gpui.review.gnosisdev.com/tx/0xc2eb04b549d68c770062591e2561c5b1534ea665c6179d33a779c52376c9161d/

1. Drag the center of the diagram (or any its element) outside the batch viewer area and drop it there
   AR: the element stays outside the area, and it is impossible to select it/move anywhere else unless you refresh the page: https://watch.screencastify.com/v/qw1ZJeLLR4GXubPt3h8i
   ER: it would be nice not to allow to drop a diagram area outside the natch viewer area

2. Cow icons stop to be displayed when drag the diagram. See the video: https://watch.screencastify.com/v/yaePSIrC4DQthhd2gBJw
   ER: it would be nice to keep them always visible.

3. It would be nice not to allow to overlap elements (drop one element above the other one)
   

Original issue gnosis/gp-ui#1074 by @elena-zh