Improving object_filter

[andybak]

Currently a linklist.py file has a dictionary key object_filter - which tells it which model instances it can ignore.

It currently has a couple of limitations:

1. It means both "ignore as a source of links" and "ignore as a destination of links". There might be cases where you want one and not the other
2. It accepts a dict as a parameter and basically does MyClass.objects.filter(**object_filter). I've got several Model classes where visibility can't be expressed as a simple dict. I would like to modify this class to also accept Q and maybe F objects.