New 'org-category-position text property in agendas.

New 'org-category-position text property in agendas.

* org.el (org-refresh-category-properties): New text property
'org-category-position to point at the beginning of the
headline from which the category is set.

* org-agenda.el (org-search-view, org-agenda-get-todos)
(org-agenda-get-timestamps, org-agenda-get-sexps)
(org-agenda-get-progress, org-agenda-get-deadlines)
(org-agenda-get-scheduled, org-agenda-get-blocks)
(org-agenda-filter-by-tag-refine): Add the
'org-category-position text property.

This commit prepares for a future `org-agenda-set-category'
function allowing to set the category of the headline at point.