Soporte tematizacion de subtipos

Well, the name itself, `datos_desplegable` already suggests that it is something related to a view (`desplegable` doesn't seem to be an inherent property or function of a `Tematica`). Reading it makes me think about a `select`, or list or similar thing, not about the models own attributes and functionality.

It does not change data, so it can be argued that it is not presentational, justs filters it (although a filter is also a change), but I do think that probably the best place for this isn't in the model. A decorator could be a good fit, but I would understand if it was argued even to just put it in the controller (a cursory search shows *one* use of the scope in Rankia, none in Verema), as also when reading `datos_desplegable`, my first thought is 'What desplegable? There's no one near here.'