Added function in functions/imap_mailbox.php to generate list
of <OPTION>... </OPTION> tags for mailboxes using chosen
delimiter in list.
folders page (create/remove/etc..), delete_move_next plugin, and
mailbox_display were all updated to use this function so mailbox
is displayed consistently everywhere.

Need testing by people with Unsubscribe/Subscribe folder lists,
and different (or no) delimiters.