inconsistent usage when filtering searches

[francescalb]

1. Search with filtering does not allow for full iris of the keys. This should be changed.
2. predefined keywords do not allow for prefixdefinition in search. E.g. 'dcterms:creator' cannot be used, only 'creator'. At the same time, the predefined keyword 'theme' doesnæt work in the search.
3. It is not possible to filter search with more than one value for each key. E.g. it is not possible to aks that the type should match two types not only one. Same for all other criteria.

[francescalb]

Number 3 i closed in 397

[jesper-friis]

Number 1 and 2 are closed in PR #401