TlsConnection: Add dedicated "handshake-completed" signal
Right now the only way an application is able to discover that a TLS Handshake is completed, the connection is encrypted and data can be sent is to connect to the peer-certificate property via notify::peer-certificate
This seems very prone to breakage as its probably not easy to remember for contributors/maintainers that applications depend on the exact moment a property is set, or infer other things from that moment.
Currently this seems broken see #2013 (moved)
I would suggest as an enhancement to make the moment the handshake is completed and the connection is ready to send encrypted data more explicit with a dedicated handshake-completed
signal