Added an 'undefined' option

When 'add_undefined' is set to true, the objects missing a property can be selected.
(demo here: http://centaurus-dev.eea.europa.eu/search on EEA Topic)

The URL query for the AND operation has been changed so in order to have it functional again (with 'undefined') there are several possibilities:
1. disable 'undefined' for 'AND' queries and keep the original version
2. change the URL query to the one based on filters completely
3. keep both implementations separately