Don't trigger an assert in QHeaderView for not yet existing columns.
Move instance label to base class for re-use on the client side.
Move instance label logic to the server, and fix it (was always showng pid).
Add a server info message send after connecting.

So far only contains the instance label, but can be easily expanded to also
contain additional information we need (eg. the Qt version).
Only consider a connection as established once we got all initial messages.
Bump protocol version since the client now waits for the ServerInfo message.