Fix label processing in GmailMaildir

Commit 7df765cfdb4096a33f35bf709db6820a6c2c79fd introduced regression:
GmailMaildir caches labels in its own function and it was testing the
presence of the 'labels' key in message descriptor.  But 7df765cf
changed descriptor initialization and this key is always present.

So now we have 'labels_cached' flag that tells us if labels were
already cached or not.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>