Add lilv_state_emit_properties

[rerdavies]

Allow hosts to read properties from LilvState.

[rerdavies]

So0lution for issue #57

[drobilla]

The API looks reasonable for what it does, thanks for including a test! I ran this through the consistency grinder and bumped versions and so on towards a releasable version on https://github.com/lv2/lilv/tree/emit-properties (currently ee854ef).

I'm not sure about this, though. The motivation was to copy state data, which I feel should be done with the general world API, and it exposes the structure of properties as single atoms in the API. That's more of an implementation detail in my mind, and I'm not sure what happens with complex state that has nested blank nodes, or other potentially tricky cases. Maybe it doesn't matter in practice but I'd rather not expose those assumptions in the API unless it's necessary. I'm not sure this is the best way to do what you were originally trying to do though, so I'll hold off on merging this for now and take that up in the ticket.