Move private QXmppTlsManager into OutgoingClient again

[lnjX]

Setup of STARTTLS is definitely a task for the outgoing client. My original idea was to do much more in client extensions, but I don't think that's a good idea, doing the stuff in the outgoing client directly offers more flexibility and it's okay that the core isn't completely accessible from outside (and QXmppTlsManager was private anyway). My original intention with the TlsManager was also to fix the issue that the registration manager would be used before tls management was done.