org-agenda.el: Rewrite agenda filtering using text properties

org-agenda.el: Rewrite agenda filtering using text properties

* org-agenda.el (org-agenda-write): Code cleanup.
(org-agenda-mark-filtered-text)
(org-agenda-unmark-filtered-text)
(org-agenda-fix-tags-filter-overlays-at): Delete.
(org-agenda-remove-marked-text): Use the `property' argument
instead of hard-coding 'org-filtered.
(org-agenda-filter-remove-all): Use `org-agenda-finalize'.
(org-agenda-filter-hide-line): Rewrite using text properties
instead of overlays.
(org-agenda-remove-filter): New function.
(org-agenda-filter-show-all-tag)
(org-agenda-filter-show-all-cat)
(org-agenda-filter-show-all-re): Rewrite using
`org-agenda-remove-filter'.