AdvancedSearch: Fixed bug which renders the wrong widget for first request

Since the filing number splitting, the very first requests of the advancedsearch form, renders the wrong widget for the fiels.
As a consequence, that all the javascript (hiding fields etc.) does not work.

![image](https://cloud.githubusercontent.com/assets/485755/3702184/51dc9ed8-13fc-11e4-9624-99787398906b.png)

That happens because the fields variable was saved on the class. Reworking them and define a property instead fix that bug.