Add mechanism for acquiring import(or template)-tree for a file under a context (or without)

Allow clients to request from the server the complete DAG of imports/templates for a file/template.

Considerations:
- How will this be presented in the client? This affects which information the DLS should supply
- How to handle this in common-code files not under a device (obviously, template trees are impossible here since we do not have a semantic analysis, but best-effort import tree may be possible)
- If we want to add extra information to connect sibling file/templates if they touch the same parts of the device object tree