* Automatically marked read when entering MessageView
* Marked unread via menu selected during MessageView
* Provider updated (directly - see note)
* Enable context menu in MessageList and enable "open" & toggle unread
NOTE: Does not use the correct service notifications yet; Just updates
the providers.
NOTE: The UI for the context menu is incomplete, it says "mark as read"
but it actually toggles the state. The true UI is to flip the text to
match the current state e.g. "mark as read" or "mark as unread". That
will be much simpler to implement when we switch to a custom list item
view class, where we can cache the read/unread state (and other tidbits).
