GPFILTERPANE-81 Fix multiple associations to same domain class property

Hi,

I've fixed the http://jira.grails.org/browse/GPFILTERPANE-81 based on the provided patch.
I had to make some arrangement since the FilterPaneFieldNamePrefix was the same for all associations related to a specific domain class property. I'm now using a dynamic prefix method in order to store different prefix according to the association used.