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.h

@@ -0,0 +1,25 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/private/webrequest.h
+// Purpose:     wxWebRequest implementation classes
+// Author:      Vadim Zeitlin
+// Created:     2020-12-26
+// Copyright:   (c) 2020 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_PRIVATE_WEBREQUEST_H_
+#define _WX_PRIVATE_WEBREQUEST_H_
+
+// ----------------------------------------------------------------------------
+// wxWebSessionFactory
+// ----------------------------------------------------------------------------
+
+class wxWebSessionFactory
+{
+public:
+    virtual wxWebSession* Create() = 0;
+
+    virtual ~wxWebSessionFactory() { }
+};
+
+#endif // _WX_PRIVATE_WEBREQUEST_H_