Feature: Merge request document

[mikefarmer01]

I have created multiple GraphQL request documents.
Now I'd like to create a single request document from it that combines/merges the original request documents.
Ideally, this combine/merge relationship should be dynamic so that changes in the origin request documents are reflected in the unified request document.

This could be implemented as follows:

1. Select two or more request document from the list
2. Right click -> "Combine to new"