Implement category filtering.

Implement category filtering.

* org-agenda.el (org-agenda-filter-preset): New alias.
(org-agenda-filter-by-category): New command.
(org-agenda-mode-map): Add the new command.
(org-agenda-custom-commands-local-options): Add category
filter preset.
(org-agenda-mark-filtered-text): Mark both tag and filter
overlays.
(org-agenda-category-filter-preset): New variable.
(org-finalize-agenda, org-agenda-redo)
(org-agenda-filter-make-matcher, org-agenda-filter-apply):
Handle both category and tag filters.
(org-agenda-filter-show-all-tag): Rename from
`org-agenda-filter-by-tag-show-all'.
(org-agenda-filter-show-all-cat): New function.
(org-agenda-set-mode-name): Show the category filter in the
modeline.

* org-faces.el (org-agenda-filter-category): New face.

* org.texi (Agenda commands): Update documentation about the
new category filtering feature.

This feature has been requested by several people -- thanks
to all of them for mentioning this possibility.