Fix for NB#88031:

Fix for NB#88031:
* src/modest-account-mgr.[ch]: added method to check if there's
  already an store account with the same login, hostname and port
  as the one we're trying to add to account manager.
* src/maemo/easysetup/modest-easysetup-wizard-dialog.c: now we
  check also on trying to save the account if there's already an
  account with the same setup data. We prevent as we don't want two
  modest accounts using the same cache.

pmo-trunk-r5606