Fix CMake build types and compiler flags

Turns out there were various typos in this that noone ever noticed,
most notably leading to QT_NO_DEBUG not being defined in RelWithDebInfo
builds. We're now also defaulting to RelWithDebInfo for Win32; please
let me know if that still causes problems.

While I was at it, I also added CMAKE_BUILD_TYPE=Profile, and pimped the
compiler warnings in general. In particular, -Woverloaded-virtual seems
rather useful with all the template trickery we're introducing lately.
Fix virtual overloads

So I learned something today: the 'using' keyword can also be used to
import function names from the enclosing namespace. This not only
solves the spammy virtual overload warnings that the new-fangled protocol
stuff introduced unwittingly, but also removes the need to redefine
RemotePeer::handle<>() as a forward to Peer::handle<>().

Also, someone please kick the C++ inventors for making method lookup
not follow the Koenig rules (contrary to most other lookups...).