Improve view mixin

[james-emerton]

The current implementation of FilterMixin (undocumented) is not currently usable in my own class based views without a fair bit of boilerplate. I took a stab at improving it, which I will happily reshape into a PR if there is interest.

• These changes may not be backward compatible, so the new mixin is called FilterViewMixin. This also makes it clear that this isn't a mixin for the Filter class.
• The filter context variable is renamed to filterset for consistency
• get_filterset() no longer requires the filterset_class attribute
• The filterset is stored as an attribute the first time it's created and can be reused throughout the request
• Added get_filtered_queryset() to retrieve the filtered queryset
• Replace object_list with the filtered queryset in get_context_data()
• get_table_data() is implemented for compatibility with django-tables2 SingleTableMixin

I've posted my current version as a gist: https://gist.github.com/james-emerton/8c7f23cdbe472bd6c1098692cd93436a

[carltongibson]

Hi @james-emerton — thanks for the suggestion. This isn’t something I want to look at right now, but I’ve moved it over to the discussions, and will have a think about it.