Adding some fields to mdl_event to facilitate iCalendar import/export.

This is a work in (sloooow) progress.

lib/db/postgres7.sql

@@ -126,6 +126,8 @@ CREATE TABLE prefix_event (
    timestart integer NOT NULL default '0',
    timeduration integer NOT NULL default '0',
    visible integer NOT NULL default '1',
+   uuid char(36) NOT NULL default '',
+   sequence integer NOT NULL default '1',
    timemodified integer NOT NULL default '0'
 );