Fixes issues with layer reordering

[matthewma7]

This addresses the issue https://github.com/Kitware/minerva/issues/423.

This basically implements a new logic for calculation the 'stack' value of datasets when reordering.

Issue reproduce: Add more than three datasets to session layer, reorder layer freely, the map and panel will start to behave unexpectedly.

Fix result:

Sample datasets: (Any dataset will do, These three are provided for convinience. Rename .txt to .geojson before uploading) buffer.geojson.txt intersect.geojson.txt within.geojson.txt

[matthewma7]

Screen records added. 🖼

[matthewma7]

@dorukozturk Thanks 😃