BUG GridFieldFilterHeader works without non-filterable cols

Previously relied on the presence of a last column which wasn't filterable,
commonly a GridFieldEditButton. If this wasn't present, the filter buttons
were never added, leading to the GridField JS reload request being sent
without the required button form action, so GridFieldFilterHeader->handleAction()
was never called.

Raising this against 3.0 since I think its a fairly important bug: The UploadField "select file" filtering doesn't work because of it.

The filter buttons don't look that great (just tacked onto the last filter field), but they have to go *somewhere*.