[geary] (8 commits) ...Merge branch 'wip/713530-background-sync'. Fixes Bug 713530.

[Michael Gratton <mjog src gnome org>]

Summary of changes:

  4e0b1d8... Update doc comments for Geary.Folder and ::list_email_by_id (*)
  5387bd8... Don't use local/remote counts to determine if folder should (*)
  8db48f8... Ensure the account synchroniser actually does some vector e (*)
  a74dfb5... Ensure duplicate messages are still added to local folders. (*)
  f55cdc3... Add some debugging so we know when a background sync has co (*)
  926aae4... Enforce a an API contract clause for Geary.Folder.list_emai (*)
  7c5d89b... Only prefetch messages that have not been seen before. (*)
  b1d3d49... Merge branch 'wip/713530-background-sync'. Fixes Bug 713530

(*) This commit already existed in another branch; no separate mail sent