Ensure that RT::Ticket has a method for every role group

Roles are in the singular declension; thus support and encourage
->Requestor for consistency.
Allow RoleGroup to apply an ACL before returning a loaded Group

This turns ->AdminCc and friends into simple wrappers around the
respective RoleGroup methods, so they can be used interchancably.  This
nudges tickets slightly closer to being able to have configurable role
groups.
Move $linkUsers into an element so it can be used externally

Multiple locations, particularly where role groups are involved, may
wish to enumerate and link group and user contents.
Add role groups to ColumnMap computationally

If a class ColumnMap does not explicitly define handlers for its role
groups, compute and display them computationally.  Unfortunately, $Class
is already in the form RT__Ticket and not RT::Ticket, and thus must be
computationally transformed back.
Move ShowGroupMembers to new ShowPrincipal method

Both accomplish identical purposes; happily ShowPrincipal gives
sufficient rope to re-implement ShowGroupMembers.