Prevent editing of 'special' case activity types : Change Case Status, Change Case Start Date, Change Case Type

Refer to item '18 (b)' in CiviCase Phase 3 wiki page:
http://wiki.civicrm.org/confluence/display/CRM/CiviCase+-+Phase+3+Candidates

I'd prefer if we can control this via a new element in the CiviCase settings file : CRM/Case/xml/configuration/Settings.xml, something like this (feel free to modify structure if there's an easier approach).

<Settings>
...
<ActivityTypes>
<ActivityType>
<name>Change Case Status</name>
<editable>0</editable>
</ActivityType>
</ActivityTypes>
...
</Settings>

If this doesn't make sense, then I guess we can hard-code the 'list' for now (or add to civicrm.settings.php file ?? ). I'm a bit concerned that different orgs will want different behaviors (i.e. ok to edit any activity type, or a different set of activity types that they want protected) - so I think we need flexibility here.