Add RelatedAllFilter?

[rpkilby]

A current limitation is that you cannot use AllLookupsFilter in conjunction with RelatedFilter. The following obviously does not work, as the declared filters will overwrite each other:

class FooFilter(FilterSet):
    bar = RelatedFilter()
    bar = AllLookupsFilter()

    class Meta:
        model = Foo

You cannot use the '__all__' shortcut for the dict-style syntax either, as it's just a shorthand for the declarative style above.
edit: the next release of django-filter will allow '__all__' to be reimplemented, solving a couple of issues. This still doesn't fix the declarative case though.

A workaround is to use the dict-style syntax without '__all__'. eg,

class FooFilter(FilterSet):
    bar = RelatedFilter()

    class Meta:
        model = Foo
        fields = {
            'bar': ['exact', 'lt', 'gt'],
        }

Including 'exact' does not matter here, as generated filters will not overwrite declared filters (which would be the RelatedFilter in this case.

We could add a RelatedAllFilter, AllLookupsRelatedFilter, AllLookupsWithRelatedFilter, or something along those lines that combines the behavior of both AllLookupsFilter and RelatedFilter.

[rpkilby]

The solution adds a lookups argument to RelatedFilter. It expects a list of lookup expressions, identical to the values for the Meta.fields dict syntax.

class FooFilter(FilterSet):
    bar = RelatedFilter(filterset='BarFilter', lookups='__all__')

[rpkilby]

@jsmedmar. I'm contemplating reverting #114 in addition to deprecating AllLookupsFilter (see #121). Going forward, the following would be the correct usage:

class FooFilter(FilterSet):
    bar = RelatedFilter()

    class Meta:
        model = Foo
        fields = {
            'bar': '__all__',
        }

Any thoughts?

[rpkilby]

Same reasons as in #121 - it makes sense to leave this in.