Append to merge two graphs delete nodes

## Expected Behavior
If I open a first graphml (or gexf) graph in new workspace, then open a second graphml (or gexf) and choose "append" so as to merge the two graphs, then I would expect the nodes of the second graph to be added (and probably renamed) to the first graph, and the edges to remain for each graph (if I chose the strategy don't merge for the edges).

## Current Behavior
Actually, what happens is that the second graph deletes the nodes of the first graph: the nodes of the second graph take the place of those of the first.

Hence it is impossible to merge different graphs in Gephi.

I am opened to any solution to allow for being able to merge graphs (note that the issue of nodes being deleted when copying from one workspace to the other stops using this method to overcome the proble)

## Possible Solution

## Steps to Reproduce

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## Context

I need to have different people working on different graphs and then to be able to merge their work/the various graphs.

## Your Environment

* Version used: Gephi 0.9.4
* Operating System: MacOS catalina 10.15.7