New defun `org-refresh-effort-properties'

New defun `org-refresh-effort-properties'

* org-agenda.el (org-agenda-format-item): Do not use
org-get-effort to get the effort text property.
(org-agenda-get-sexps): Use `org-back-to-heading' when setting
the tags.

* org-clock.el (org-clock-in): Refresh effort properties.

* org.el (org-refresh-effort-properties): New defun.
(org-get-effort): Delete.
(org-set-effort): Set the 'org-effort text property.
(org-property-next-allowed-value): Ditto.
(org-agenda-prepare-buffers): Refresh effort properties.