[buteo-sync-plugins] Improve functionality for importing contacts via Bluetooth

- Use a "bluetooth" syncTarget to allow the contacts backend to avoid
  clobbering existing local contacts with data from a bluetooth-imported
  contact.

- Save the MAC address of the Bluetooth device that sent the contact in
  the QContactOriginaMetadata of the contact so that if we import from
  the same device later, we know we can safely overwrite any existing
  details.

- Add ContactImport implementation to allow existing contacts to be
  updated. We can't just clobber existing contacts completely as per
  the previous implementation as this will cause user-applied links
  to disappear.

- Apply the "slow" sync mode if this is preferred by the profile. This
  allows the client to request all contacts from the other device even
  if the other device has not made any changes to their contacts. This
  is the behaviour we want for an import-style operation.