Cleanup unused Session::network
Move the ConnectionInfo converters from Session to ConnectionInfo

In QML, Session* and IrcSession* are not compatible property types,
so the ultimate goal is to get rid of the whole Session class.
Remove Session::secure - it was promoted to IrcSession
Update libcommuni
MenuFactory: kill deps to Session::userPrefix() & unprefixedUser()
Cleanup obsolete Session::userPrefix() & unprefixedUser()