Bug 800402 - [email] UI: Show the number of unread messages in the message-list header after the folder name

This should be marked as optional and probably be more like "For ActiveSync folders, the server-issued id for the folder that we use to reference the folder."  When we throw "UID" around through the code, it's from the IMAP spec's explicit concept of a "UID".  Sadly, when used for our slices/etc. we really should have renamed it to "ID" since it usually isn't a UID anymore but an id we assign.