org-agenda.el: Fixes for top headline filtering

org-agenda.el: Fixes for top headline filtering

* org-agenda.el (org-agenda-local-vars): Add
`org-agenda-top-headline-filter'.
(org-agenda-filter-by-category): Fix syntax.
(org-agenda-filter-show-all-top-filter): New function.
(org-agenda-filter-by-top-headline)
(org-agenda-filter-remove-all): Use the new function.
(org-agenda-filter-top-headline-apply): Use a dedicated symbol
`top-headline'.

Thanks to Greg Troxel who provided a preliminary patch.