Before, we were using several IMAP-specific concepts to represent
information being displayed in the UI. Factor these structures out of
the IMAP package to make it easier for other backends to provide the
required information.
Consists of 3 functions
* Store: Access to MessageStore type
* SelectedAccount: Access to Account widget that the target widget
belongs to
* SelectedMessage: Current message (selected in msglist or the one we
are viewing)
Signed-off-by: Kevin Kuehler <keur@ocf.berkeley.edu>