API for querying the compression status of a QSslSocket

The QSslSocket offers an API for enabling and disabling of the transparent SSL compression. What is missing is an API for querying whether the compression has been already negotiated.

This is needed for example for an efficient support of IMAP. In IMAP, the network transmissions can be compressed either via the TLS layer or explicitly by the application code using zlib. It is expected that clients will prefer the SSL compression and only activate the explicit higher-level one when the SSL compression is not available for some reason. In the real world, most of the servers support this, but there are certain scenarios (like a farm of IMAP servers behind an SSL accelerator) where the IMAP server cannot tell whether the compression is active already, and the client using QSslSocket currently doesn't know either. The ability to query the socket whether the compression has been negotiated would therefore be very useful.

I've talked about this with Rich Moore and he has encouraged me to file this bug so that this feature request doesn't get forgotten.