Adding operation-related search criteria

The idea is to be able to add a mix of operations when calling the q (query) webservice.

As a result, it is possible to the which MDs the user is able to view|download|editing|notify|dynamic|featured, the parameter can also be repeated, e.g. for visualizable AND downloadable metadatas, the call would look like:

```
  q?operation=download&operation=dynamic
```