Move selection-related logic out from the core onto middleware

[ahitrin]

Selection-related logic was in the core of SiebenApp (domain.py, goaltree.py) since the very first versions. But now it looks obsolete.

We should move it from domain.py and goaltree.py onto a new persistent middleware layer. Our goals are the following:

1. Simplify logic inside goaltree module.
2. Simplify tests (and remove irrelevant ones).