Refactor the transfer stuff

Well, sometimes refactorings come sooner than usual. While thinking
about the next steps, I decided to handle the actual transfer logic
inside Transfer rather than TransferManager - now that we can have
selective (peer-based) sync calls as well, that makes more sense than
outsourcing that to TransferManager. Thus, it now makes lots of sense
to have ClientTransfer and CoreTransfer, as the logic will be quite
different.

I also took the opportunity to clean up the accessors and make them
safer; you can now get a non-const CoreTransfer pointer, for example,
but still only a const ClientTransfer pointer.

Also, some useful signals have been added.