Copying and pasting compartment in Lua invalidates mapping snapshots

[helgoboss]

The issue is that mapping snapshots internally use the technical (non-persistent) MappingId as qualifier. These IDs change when importing the mappings via Lua compartment import, even their (persistent) keys are the same. When exporting the session again (via JSON), we see that all snapshots are gone.

[helgoboss]

I'm fixing this by changing the key-to-ID translation process (part of data-to-model conversion context) to first have a look into the session (if it already has an ID for that key) instead of always generating new random IDs. Then we don't have to touch the snapshot container structure when importing the compartment - which is clean because the snapshot container structure is not part of a compartment (it's part of the session). Also, this will fix potential other issues and future issues with mapping IDs that could get orphans.