Apart from the semantically different content types, I would suggest that this is the same type of API call for Manticore to implement, as already suggested in T725#10210.

While Manticore would concern itself with "documents" -- of whichever type the editor supports --, it may let the client (chwala, roundcube's plugins) concern itself with what (type of documents' sessions) to display where and how (i.e. application operating context and icon).

Should a concern become, a call to /sessions/list becomes to large a list for a client application to handle in aggregate, then a /session/list?type=note could be added for the list appropriate for the contextual view in the consuming application.