external model interface

[Ichthyostega]

factor out a export interface of the high-level model, cleanly separated from the implementation.

Ideally, it should be usable without linking against liblumierasteam.so

[Ichthyostega]

(in #44) Meanwhile, I'm understanding the situation much better.

• I was able to remove all dependencies from lib. This essentially breaks the cycle between the libraries.
• but the common (application core) will always necessarily depend on both steam and vault. The reason are the data types used on the Session and Engine interfaces.
• if we want to avoid the latter, we'd need to do some magic dance with an transparent proxy layer and export interfaces using only placeholder data types.

I don't see any reason why to put any effort into the latter right now. Thus closing as wontfix

[Undercover Agent]

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