Rename Internal-, Remote- and LegacyConnection to -Peer

While I'm not tooo happy with the word "Peer", I couldn't think of another one
that fits their purpose better. "Connection" was certainly less correct, and
we have so many different connections in the codebase that it got way too confusing.

At least, while Peer might not be a perfect match, it is consistent with how we always
called that in Quassel (and they still inherit from SignalProxy::AbstractPeer and have
been doing so for years!).