Allow DBList to retrieve data from other wikis

Use case: Have a Product class in a wiki and use Categories from another wiki.
We can provide support for 2 situations:
1) When the XWiki Class Name property is a full reference, containing also the wiki (xwiki:XWiki.MyClass), then the query should be executed in the class' wiki.
2) Add a new metadata Wiki to the DBListClass to specify the wiki and use that value when setting the wiki in the DBListBuilder.