Make wxWebSessionFactory private

Having wxWebSessionFactory part of the public API implies keeping
compatibility with the possible ways of implementing it which is too
restrictive for no good reason, so move this class to the private header
and don't document it nor wxWebSession::RegisterFactory() (which is now
private).

include/wx/private/webrequest_curl.h

@@ -12,6 +12,8 @@
 
 #if wxUSE_WEBREQUEST_CURL
 
+#include "wx/private/webrequest.h"
+
 #include "wx/thread.h"
 #include "wx/vector.h"