Ensuring f-c-a is fired means that the background sync will pick up the
changes from the operations after they happen, rather than when the next
folder refresh happens.

* src/engine/imap-engine/imap-engine-generic-account.vala
  (Account::update_folders): Provide a convenience singleton version of
  ::update_folders to fire folders_contents_altered.

* src/engine/imap-engine/imap-engine-generic-account.vala
  (RefreshFolderUnseen): Update cached status values for a folder and
  fire f-c-a signal, but only for folders that have changed.

* src/engine/imap-engine/imap-engine-minimal-folder.vala
  (MinimalFolder::copy_email_async): Fire f-c-a signal after the op
  completes.

* src/engine/imap-engine/imap-engine-revokable-move.vala (RevokableMove),
  src/engine/imap-engine/imap-engine-revokable-committed-move.vala
  (RevokableCommittedMove): Notify contents have changed when a move is
  revoked.