Refactor wxLog and wxMessageOutput classes to avoid duplication

Add wxMessageOutputWithConv mix-in class to avoid duplicating the same
code in wxLogStream and wxMessageOutputStderr.

Also derive wxLogStderr from wxMessageOutputStderr to reuse its code
without having to create a temporary object of this type (which will be
more expensive now that doing it involves creating a heap-allocated
conversion object copy).

include/wx/log.h

@@ -61,6 +61,7 @@ class WXDLLIMPEXP_FWD_BASE wxObject;
 
 #include "wx/dynarray.h"
 #include "wx/hashmap.h"
+#include "wx/msgout.h"
 
 #if wxUSE_THREADS
     #include "wx/thread.h"
@@ -716,34 +717,31 @@ private:
 
 
 // log everything to a "FILE *", stderr by default
-class WXDLLIMPEXP_BASE wxLogStderr : public wxLog
+class WXDLLIMPEXP_BASE wxLogStderr : public wxLog,
+                                     private wxMessageOutputStderr
 {
 public:
     // redirect log output to a FILE
     wxLogStderr(FILE *fp = NULL,
                 const wxMBConv &conv = wxConvWhateverWorks);
-    virtual ~wxLogStderr();
 
 protected:
     // implement sink function
     virtual void DoLogText(const wxString& msg) wxOVERRIDE;
 
-    FILE *m_fp;
-    const wxMBConv *m_conv;
-
     wxDECLARE_NO_COPY_CLASS(wxLogStderr);
 };
 
 #if wxUSE_STD_IOSTREAM
 
 // log everything to an "ostream", cerr by default
-class WXDLLIMPEXP_BASE wxLogStream : public wxLog
+class WXDLLIMPEXP_BASE wxLogStream : public wxLog,
+                                     private wxMessageOutputWithConv
 {
 public:
     // redirect log output to an ostream
     wxLogStream(wxSTD ostream *ostr = (wxSTD ostream *) NULL,
-                const wxMBConv &conv = wxConvWhateverWorks);
-    virtual ~wxLogStream();
+                const wxMBConv& conv = wxConvWhateverWorks);
 
 protected:
     // implement sink function
@@ -751,7 +749,8 @@ protected:
 
     // using ptr here to avoid including <iostream.h> from this file
     wxSTD ostream *m_ostr;
-    const wxMBConv *m_conv;
+
+    wxDECLARE_NO_COPY_CLASS(wxLogStream);
 };
 
 #endif // wxUSE_STD_IOSTREAM