Cleanly separate GUI socket-related code from net library.

Cleanly separate GUI socket-related code from net library.

This fixes linking problems under Unix introduced by recent changes which
fixed previous problems which were due to files not being linked in at all.

In order to provide a clean separation between base, net and core libraries we
now use the same wxSocketManager (wxSocketFDBasedManager), defined in net
library for both console and GUI Unix applications and just use different FD
IO manager for them: the latter can be defined in base and core libraries as
it doesn't involve wxSocketImpl at all, only its base wxFDIOHandler class.

At more detailed level, these changes required:
 1. Adding the new wxFDIOManager class.
 2. Refactoring the old (and now removed) wxSocketFDIOManager to use the same
    code as wxSocketFDIOManager. This involved:
  a) Adding handler and direction parameter to RemoveInput().
  b) Storing the mask of registered events in wxFDIOHandler itself.
  c) Defining wxFDIOManagerUnix which works with wxFDIODispatcher.
 3. Changing the traits classes in Unix ports to define GetFDIOManager()
    instead of GetSocketManager().

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61688 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775