Add filter to map

[Discostu36]

A filter menu should be added to show different types of objects on the in the data layer (#3):

Which kinds of objects are relevant enough should be dicussed in a separate issue (#31).

[Discostu36]

The problem of adding this feature so early is that it makes the automatic creation of Wikimedia Commons descriptions and categories more complicated, as it needs to be done for all the different types of objects. So this will be delayed to a later milestone and the app will only work for buildings in the beginning.