Allow peer-specific sending and receiving of remote signals

Allow peer-specific sending and receiving of remote signals

For file transfers and possibly other use cases, it is important to know
which peer has received a specific signal, and to allow sending remote
signals only through a particular peer rather than multicasting to all connected
clients.

For this reason, we now treat attached signals/slots specially if their first
argument is a PeerPtr (a typedef for Peer *):

* An attached slot will have a pointer to the peer which received the signal in
  its first argument
* An attached signal on the core side will be relayed only through the peer that
  is given as the first argument (and the receiving slot will have a pointer to
  the client-side peer in this argument)
* The peer argument for an attached signal on the client side will be ignored,
  as clients only have one peer anyway