Meta: Deal with pagers and sorting

Known Issue: Use a pager and sort by a field that is not one of the selected row or column header fields.

Possible solution: Implement views_plugin::build_sort() and always sort by the row or column header fields. This would cause less confusion if someone added a sort to the view because it will be sorted that way anyway. It would also fix the pager issue somewhat.

Some logic could be added by implementing views_plugin::query() that removes pagers in certain conditions too. Not sure which I like more.