Use SQLAlchemy's Declarative system

[ockham]

In order to get rid of some database/ORM related overhead, we should consider to switch from SQLAlchemy's "Classical" Mappings to its Declarative system. For an intro, see e.g. here.

BTW, this might be a first step towards a programming paradigm like MVC (Model/View/Controller).

Some preliminary experiments suggest that this is quite invasive as it also entails a lot of other changes to the database backend, which makes it error-prone. It will thus have to wait for a post-0.16 release.