Adds UI modification (appeareance) for EmailView of seen emails.
EmailList can be populated with both seen and unseen emails if required.
Implements showSeen menu toggling and its effect on the emails displayed for each account.
Implements Account deletion with a dialog for confirmation.
Implements a Message comparator to order emails in reverse chronological order by SentDate.
Streamlines email deletion by removing EmailView removal from the deletion loop and simply refreshing the entire Fragment view once all emails marked for deletion have been deleted.