[Nice] exposing streams and components in the API

[Dafydd Harries]

When I designed the agent API, I made the decision to hide the
stream/component structs and have them be referred to only by ID. Looking over
the code again, I'm having second thoughts about this. It means that we need
to call agent_find_stream/agent_find_component in a bunch of places (10 and 18
respectively in the full mode branch). It makes it harder to split agent.c
into smaller logical files.

Exposing the stream and the component might complicate the API somewhat. It
might involve adding pointers from Component to its stream, and from Stream to
its agent, which is a bit smelly.

Thoughts?

-- 
Dafydd