Expose ALL activity types to Scheduled Reminders Entity select

Currently the system-generated 'component' activity types are suppressed in the list of activity types you can use to trigger a scheduled reminder. However, these can be useful and there doesn't seem to be any reason not to include them in the list of options.

See forum discussion:
http://forum.civicrm.org/index.php/topic,30793.0/topicseen.html