How to set "Add new value(s) to FIELD, instead of overwriting the existing values. " a

Problem/Motivation

In our use case we want regular user to be able to add terms but not delete or overwrite.
We fixed this in code in this issue for d7
https://www.drupal.org/node/2046653 →

The nicer solution would be to allow the site builder to choose in the view setup which options that would be available for the user to select.
It would be nice if this could be integrated

In our use case:
Permission on the field itself is not a solution.
The regular user can add, delete, modify tags on a per node basis, so permissions are granted.
The regular user can add only using vbo (protection for changes beyond there intention or comprehension)
Only curaters can use the more dangerous delete and overwrite on vbo site wide scale.

Proposed resolution

In regular VBO without the patch the assignment overwrites al previous present values in the field with the new value.
It is not possible to add a value to a field over a selection of different nodes

With the patch the following is possible:
Start state:
node1: A
node2: B
node3: C

node 2&3 selected assign K
result:
node1: A
node2: B, K
node3: C, K

node 1& 2&3 selected assign L
result:
node1: A, L
node2: B, K, L
node3: C, K, L

We still keep the initial values of A, B and C in there respectable nodes, unaffected by the manipulation.

Remaining tasks

port/ integrate in this module instead of vbo? or better a vbo feature?

User interface changes

read https://www.drupal.org/node/2046653 → for all info and screenshots of d7 solution