Speed up setting/getting the APPT_WARNTIME property.
* org.el (org-refresh-properties): Rename from
`org-refresh-effort-properties' and use two new parameters.
(org-agenda-prepare-buffers): Use `org-refresh-properties'.
Also refresh the 'org-appt-warntime text property.
* org-clock.el (org-clock-in): Use the renamed defun.
* org-icalendar.el (org-icalendar-print-entries): Refresh the
'org-appt-warntime property.
(org-icalendar-print-entries): Do not use `org-entry-get' to
get the "APPT_WARNTIME" property value.
* org-agenda.el (org-agenda-get-timestamps)
(org-agenda-get-sexps, org-agenda-get-deadlines)
(org-agenda-get-scheduled): Ditto.