Initial support for syncing Gmail labels

Labels are fetched and saved in user-defined header. No filtering, no
decoding, no translation.

GmailLabelsFolder implements label syncing in a generous manner: whenever
labels for the message are changed, the UID of the message is updated on
Gmail's IMAP server.

The labels are added to headers in the backend, so the implementation doesn't
require any specific support for labels in the code of the other backend in
of account, allowing Gmail<=>IMAP syncing.