Completely split apart wxWidgets and Boost.Asio networking

Completely split apart wxWidgets and Boost.Asio networking

aMule built with wxWidgets networking should not include anything from Boost.Asio
code, and vica versa (well, that's not true, detailed below).

The only valid place to #include <wx/socket.h> is in the socket code wrapper,
LibSocket.h. The only exception from this rule is amule.cpp, which includes
wx/socket.h unconditionally. This is required, as we use wxHTTP for the HTTP
download thread, which in turn uses the wxWidgets socket code. Therefore we
must initialize wxWidgets networking even if we use Boost.Asio for everything
else.

I decided to comment out the SetEventHandler() and SetNotify() calls in ASIO builds,
they do nothing, and this lets us get rid of the event handlers themselves. Also,
adding dummy MULE_SOCKET_*_FLAG definitions to LibSocket.h would clutter it too
much for nothing (since the function would be just a do-nothing in ASIO).