Make ItemSync available as public API:

- change id -> item mapping from usage of remoteId() to id() so clients can
  use ItemSync to create new items in Akonadi without having to fake a remoteId
- install the header
- remove the API dox declaring it as internal, also remove the comment about transactions
  since it is back to being a transaction.

@volker: "local" and "remote" should probably be renamed

svn path=/trunk/KDE/kdepim/akonadi/libakonadi/; revision=772470