Add support for Source in SetFilter method.

[andyneil]

It would be useful to have the Source document (when it's a context app) when applying the filter using SetFilter as is available in the SetVisibility method.

This use case here is that there may be a sub-set of records (not related to the document itself, but to a enumerable or ID property on the document)

[mattbrailsford]

The problem here is that visibility is handled in process, where as the filters are direct query where clauses. In order to perform opperations across other entities, you start to get into the realms of performing joins on the data, or, moving the filtering in process and then it becomes a resource issue to load all entities in order to filter them in memory.