UX improvement: allow to open related files on other nodes, too

[ReinhardKeil]

When editing a *.csolution.yml it is possible to open related project files.

This works fine for project and layer files.

However it fails on layer files, if the path is defined using a variable, for example:

 layers:
    - layer: $SDSIO-Layer$
      type:  SDSIO

This feature should be therefore extended with a capability to extend variables and access sequences.

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Also there are many other places where files are referenced. We should consider to extend this feature to other YML nodes.

[ReinhardKeil]

@edriouk this is the ticket I discussed yesterday. The access to variables might be best with the RPC method.

But maybe @mguzmanm can explore the many other places where files are referenced in YML nodes.