Refactoring: allow more download queries

[stheid]

Right now there is a Mediathek wrapper and a wikipedia wrapper.
Gernerally for each kind of collection there should be:

• a Query class, that takes care of doing the initial filtering and is very close to mediathek
• a Transformer class that takes care of more advanced deduplication, file renaming and so on. In the Tatort case Closely related to the Wiki data.

The plan is to create Query and Transformer as a general interface and make the current classes implement the interface. this allows for easy extension to collect other data from the mediathek.